A 35" tire on a 15" wheel will work well in the same space as a 35" tire on a 20" wheel...there will be no real difference in terms of what will fit on your rig with a given lift/set of fenders/etc. You will see a difference in ground clearance and tire performance - but that's as much dependent on rim width, wheel type, and how much you air down as it is on rim size.
17" is what most JK guys seem to run with their 35's. Anything from 16" to 18" wheels will probably work fine, but running 17's means that you'll probably have an easier time finding tires. Some people that have tried to run 15" wheels on a JK have run into problems, but it worked just fine for other people. It's a little bit hit or miss.
I'd guess that Rugged Ridge says their 4" lift will only fit 35's is that you would need to do some fender trimming (or pinch seam trimming) to fit 37's (the next step up in common tire sizes)
Many people here will agree that for technical wheeling in Alberta, what you want is the least amount of lift that is needed to run the size of tires you are going to run. Lift does not get you ground clearance - tire size does. Lift gets you two things: room to run bigger tires (good!) and a higher center of gravity (bad!)
I run a frankenlift cobbled together from different manufacturers....but I am very pleased with it. OME shocks and coils, JKS track bars, Currie control arms and coil retainer clips, JKS bumpstop extensions, and Crown brake lines. Other club members have had good luck with the same lift, and also with the AEV lift. The AEV will probably give you better on-road manners, but not quite as much off-road performance.
To be honest, what you want your Jeep to do for you is far more important than the experience that I or anyone else has had with our super-cool lift. What works well for me may be awful for you...as an example, my girlfriend loved driving my Jeep before I lifted it, now she hates driving it.
Your stock Rubicon will already go a LOT of places. Get out and enjoy, go on open runs, and talk to people to learn what's worked for them. That way you get to actually see what works on the trail instead of listening to web wheelers like me that only get our lives together enough to wheel a half-dozen days a year.