Full disclosure: I've never owned or even ridden in a SxS. However, I have run a variety of trails with them in my group. Based on that experience as well as hearing owners rave about them, here are the highlights of each.
SxS pros:
- can travel faster over mild terrain, such as fire roads
- can fit in narrower areas if you buy one small enough
- smaller and lighter means they can be hauled on a smaller, lighter trailer
Jeep pros:
- can be completely protected from the elements
- A/C
- legal to drive on every street, highway, interstate, etc in the country
- far better at tackling challenging terrain
- more cargo capacity, which makes them better for multi-day trips
Off hand, I'd say it all comes down to what you like to do off pavement. For shorter trips over easy terrain, they might make sense. Myself, my preferred style of wheeling means they are a poor choice. (Yes, I had the extremely unfavorable experience of following a 4seat SxS on the Hanging Tree Trail, and it was even worse than having an underbuilt JKU in our group.) It baffles me how much money people will spend on a RZR4--it is almost as much as a new JKU.
But, hey, to each their own.