My recommendation: get both!
I have the 686, have owned a 586, and I have the SP101. The 686 is the PLUS model with 7-rd cylinder and 6" barrel. My SP101 holds 5 rds and has the 3" barrel. Obviously, each has it's own benefits and drawbacks.
The GP100 is available in many barrel lengths and holds 6 rds.
You need to look at all the options each model offers and any aftermarket accessories you may want to add. Some accessories like holsters are very common in 4" and 6" sizes, but a 3" barrel limits your available options. Same hold true for the 7-rd cylinder. Not all manufacturers that make speedloaders offer them to fit the 686+ 7-rd models.
So, look at capacity, barrel, sights, accessories, grips, etc. Make a list of all your preferences. THEN go shopping. You'll see it's very difficult to find every one of your top choices available at all local and online retailers. an LGS can order what you want, but special orders can be more expensive.
I bought my 686+ locally when it went on sale. I bought one SP101 brand new online during a sale, and the local FFL I shipped it to told me I paid well below their cost. My second SP101 I bought used.
Do some "window shopping," prioritize your preferences, and pick up the first one you come across at a good price. You might even run across an earlier model revolver in brand new condition. Many seek out the pre-lock era revolvers, as it has been reported some actually have had the built-in locks engage while shooting. That could be a bad thing!
Have fun!