Just FYI, but you can always send your slide away for an optics cut.
JL did two of my VP9s (i sold one of them to a friend). when i got my cuts (different years) it cost me $50 for the cut itself. it was only $50 more if you wanted the slide cerakoted. he regularly does sales; the most recent one was at the start of the COVID-19 lock down. My optics sit lower on my slide compared to the guns that use an adapter plate from the factory (i.e. Glock MOS, VP9 Optics Ready, etc.). here's his website:
https://jlmachiningworks.com/of the ones you're considering, I have a p320 RX compact, a glock 17 gen 3 (non MOS), and a VP9. i haven't tried the g17 with an optic yet, but my slide is cut to accept it.
my general thoughts:
- the ergos on the glocks kind of suck in general, but the gen 5 is definitely an improvement over the gen 3 and 4. the vp9 has the best ergonomics by far. the p320 has a near limitless potential for improved grip because of the modularity. i currently run mine with a sig x-carry grip module, but there are at least a few aftermarket grip module manufacturers joining the market. even wilson combat makes grip modules for the p320 now.
- the vp9 is slightly more snappy than the p320, but barely noticeable unless you're shooting them back to back. the glock 17 isn't very snappy, but that could be bc i was using a G17L slide and barrel.
- sig and HK mags are expensive, but they occasionally go on sale. you just need to wait for the deals. I have a ton of 10-round HK mags if you're looking to buy some. glock mags will always be cheaper than the sig and hk mags.
- no matter how much you upgrade the glock trigger, it will still feel worse than a stock vp9 trigger. you probably will not notice it much until you get more accustomed to shooting handguns. some people don't care and aren't trigger snobs whereas some people are. some people are such good shooters that they will shoot fine with a stock glock trigger. however, keep in mind that the gen 5 glock trigger definitely feels better than the gen 3 and 4.
- glocks are very customizable. i started with a serialized/registered stripped glock lower (not P80) and bought all of the parts individually to assemble the whole gun. having never worked on glocks before, i can tell you that they're impossibly simple to assemble. keep in mind that i've also detail stripped my vp9, which is a pain in the ass. you can really see where the HK engineers put real brain power into designing that gun, which is part of why it has such a nice factory trigger.
- keep in mind that there are a lot of vendors selling out-of-spec glock parts, or parts that really have no purpose other than looking cool. if you go down that route, you can possibly end up with an expensive gun with very little improvement to performance.
- the aftermarket for the p320 is growing substantially. if looking cool is important to you, then you will soon have a lot of options.
- it always surprised me that glock didn't make a competition model. sig has their x-five, x-five legion, and (soon to be released) x-five alpha. HK has made competition guns in the past with their USP line and they have a long slide kit available for the vp9. glock has their 17L and their 34, but they both seem like a really low effort from the company.
in general, i recommend that you buy a full size pistol. anything in the compact category will be more difficult for you to shoot. you can always buy a compact pistol later when you are better at shooting pistols. the additional weight of a full size will help with the recoil and the added grip size will help with controlling the gun. i wear an XL size glove and my pinky falls off the bottom of a glock 19. keep in mind that the first brand new pistol i bought was a vp9 and i'd probably do it again.