With a 16" lightweight profile barrel, I can get either carbine or mid-length... design? Profile? Layout?  Contour?

As far as I can tell, the difference is only in the positioning of the gas port.  With the mid-length, the port is maybe 2" farther from the chamber.

If I end up using a railed handguard and putting my front sight there, then it seems the location of the gas port makes no difference.  But, if I decide to use the traditional gas block mounted front sight post, the mid-length gives me a (slightly) longer sighting radius. 

And, obviously, I have to buy the proper length gas tube, but that's easy enough.

So, weigh in!  Carbine or mid-length, and why?

Mid-length. The distance between the gas port and the muzzle end is the same between the 20" barrel M16 and the 14.5" military M4. If you are using a carbine length, the distance is longer due to the 16" barrel, increasing the pressure acted upon the BCG. More recoil, more wear & tear. I love my mid-length upper, has a little extra real estate for rail space, doesn't have the useless M203 cut in the barrel, and can mount a bayonet correctly.

The only way a carbine length works in my opinion is if you run a true 14.5" barrel, with a permanently mounted extended flash suppressor/muzzle brake for the 16" requirement.

As I said, I'm not the AR expert, but my reading and personal dealings have provided me some information. 

If you are going to make this decision now, mid vs carbine length, then you must have already decided to go with gas impingement over piston models?  I assume you are also locked into 5.56 versus another caliber?

Predetermined choices regarding basic specs should be included here.  They form constraints for discussion.  The caliber, intended use, desired attachments, etc. are all useful information, unless you are looking for generic answers that fit all circumstances  This series of posts will soon become a huge "if this, then that, otherwise something else" flow chart.  It might also cause the "I answered the question based on an assumption" argument.

For instance, a simple thing like attaching a bayonet, as nathanm14 said, limits the choice to mid-length, since short barrel carbine length rifles are illegal here.  There are extenders and longer than normal handled blades to compensate for the different lengths, but the availability of such bayonets is scarce.  Add to that, if you free float the barrel, then the combination of low profile gas block and longer hand guard changes things because you are changing or removing the bayonet lug.

Since this rifle is a weapon system, decisions in one area can affect options elsewhere.

There are differences between milspec and commercial spec gas systems, buffer tubes, and other components.  Each has variations in function and applicability.  Once you choose a standard spec, you are probably better off staying on that path.

Now, more specific to this question:
In general, I've seen many who say the carbine gas system is optimum for under 14.5" barrels, mid-length is best for 14.5" -16", and anything over 16" is best run with rifle length systems.

What I don't get, though, is why so many big name rifle makers, Colt included, make thousands of carbine length 16" rifles.  It's not just cost, since these rifles are the preferred brand for many law enforcement departments and agencies.  Colt supplies the M4 to the military, so their designs are trusted.  Colt owns the patent for the Armalite-designed AR-15/M4/M16.  My first AR-15 is a Colt LE6940 carbine length gas impingement rifle with free floated 16.25" barrel and 1" birdcage flash hider.  Maybe it's just an evolutionary decision to change nothing other than the barrel length making it legal for the civilian market?  I'm not sure, but I trust Colt to design these guns to be functional and reliable since so many professionals do, too.

In my opinion, the choice of carbine vs. mid length rests in the barrel you settle on.  If  you find the best match-grade barrel, or cheapest chrome-lined barrel, whatever to suite your needs, then start there.  The rest of the decision will not matter much unless the bayonet lug position is a deal breaker!

I have 3 AR's and starting my 4th.  My Noveske is a 14.5" mid-length,  My other 2 are 16.25" carbine length.  I haven't bought the barrel for #4 yet!  The carbines seem to function just fine!

Again, just an opinion and personal experience.

Aloha!

mid-length for supposedly better gas function.

Better than what? 16" carbine works and so does 18" mid. What is everyone talking about length to muzzle? Its length to chamber that determines gas pressure.

Better than what? 16" carbine works and so does 18" mid. What is everyone talking about length to muzzle? Its length to chamber that determines gas pressure.

Everyone?  Only Nathan mentioned "The distance between the gas port and the muzzle end".  Where did you see "everyone" mention that??

You sound hysterical!!                    

Better than what? 16" carbine works and so does 18" mid. What is everyone talking about length to muzzle? Its length to chamber that determines gas pressure.

not entirely, muzzle to gas port is also a factor as the bullet's departure from the bore will also influence pressure.
The .mil adopted the carbine gas system for the ability to afix a bayonet and attach an M203. The last time I fixed bayonet was in basic. Bayonets were there in OIF but I didn't carry them when I went on patrols etc. Instead I carried a CRKT folder and a KBar. I don't recall anyone fixing bayos either.
In my opinion, I won't use either a bayo or 203 in civie so I chose a midlength. Both systems work. I shot my M4 in sand storms etc. The only hiccup I got was that my M4 was out of the box new from Colt and the barrel nut was indexed too far, resulting in a slightly canted rail and gas tube - but it still worked.
smoother shooting? which one has a sharper recoil impulse? that's all perception and subjective. They both recoil. Obviously lighter guns will have a heftier recoil.

Xring is a long drive for you but I an show you in the store. Mid length is smoother shooting than a carbine has system

You should define what the weapon will be used for and go from there with questions.  Different uses may get different suggestions. 

I like a good lightweight barrel but don't prefer the pencil type barrels as they tend to shift POI too much for my liking from cold to hot.  I also like a midlength barrel but quite honestly I don't notice too much difference in the recoil impulse from carbine length.  I really start noticing the difference in say an intermediate length and of course the rifle length.  Those who tell you that midlength will make the rifle last longer due to less stress on parts are pretty much just regurgitating information that is really irrelevant to 99% of shooters.  The greater advantages are handguard length, sight radius, however if you run a low profile gas block these things become a moot point.  In reality my preferred gas block is the stock Front Sight Based that is shaved down.  It is the most rugged and reliable, but it takes a bit of work.  A working or defensive rifle and I prefer a fixed front sight base and fixed rear sights unless if I am running an optic that required rear folding sights for the optic to be mounted. 

Also the length of the barrel after the gas port to the muzzle does make a difference.  A big difference.  Gas port size and the Dwell time created post gas port can cause people fits when they have either have an over gassed or under gassed rifle.  Beware of info when trouble shooting this platform.  As you mentioned there is a ton of bad info out there.  How this platform runs and especially when it comes to diagnosing a problem and the information that is often provided on the www is even worse. 

I think the mid length gas system for a 16" barrel is a good setup.

I even choose mid lengths for my 14.5" builds.

There are some really good deals out there for complete upper receiver groups, in whatever flavor you like.

Probably cheaper than what it would cost you to roll your own.