I went with 15's for a combination of reasons but mostly price.
I don't mind the roll in corners. To be honest, I don't feel too bad in corners at all.
More sidewall offroad. I love the feeling of 35's on 15's under 15 psi!
Little easier to resale used 15" mud tires. I like to sell tires when they are about 50% so I can put that money towards a new set instead of running them bald and paying full price for a new set
Not a fan of huge rims. To be honest, 16" or 17" would have looked just as good but I am not a fan of 18"+ rims on any vehicle
BIggest reason was cost. I was buying to much crap at once. At Discount tires, my 35" BFG's 15's list price=$190, 17's=$267. Thats a $385 difference right there, could get some sliders or something with that!
BTW, to answer the OP's question, 15's WILL fit, but not all 15's. YOu will need to research each model. Backspacing and such is important but the design and shape of the inside of the wheel is really what you need to look at. For example, here is one situation MichaelW had where the rim deminsions fit but the design of the back plate didn't: CLICK HERE
Most likely, you will end up needing to run steel wheels to fit 15's on with the size of the front calipers because the material is thinner then say a forged or composite wheel.
Thanks for all the info guys, and yes you go with 15" wheels for more sidewall which equals the ability to run less tire pressure and still maintain your bead. Which in turn equals better grip off road in most situations.
ps. oh yeah going with 15" rims also saves close to a hundred bucks a tire on 36X13.5 iroks.
I'm running 15" Procomp 8179's with 35X12.5 Goodyear MTRs on my 2 door Rubicon. They fit out of the box. I chose the 15" rims for the added sidewall and tire flex when aired down. I like the combination very much.