Well, I have had both....and my favorite for all types of riding is the R1. The fun factor is just greater in the R1 (at least for me), especially in the dunes. Kind of the difference between riding a Banshee or a Raptor, I guess.
R1 power: The R1 loves to make power around 11k - 13k RPM. The sound when it's on the pipe is like nothing else, and it's just plain FAST. However, I can lug my R1 around with regular stock Rhinos all day.....the lower end power is very usable and surprises a lot of people. I don't run paddles at the dunes and can climb any hill, run the drags, and can bowl with the fastest rails without a problem. If you want the R1 between your seats in the stock location, you either need a widened Rhino or will need to do a little widening to get it there. You do have another option to place it farther back, and you can leave your seats in their stock location - we have done this for several customers and it's turned out nice.
SV1000 power: The SV has less overall HP than the R1, but you don't really notice it until you get into the top gears. It's got a very linear powerband (as do most v-twins) and it's got plenty of it for the dunes, hills, etc. Top speeds are not as fast as the R1, but honestly, who's going to be running over 100mph in their Rhino much anyway?? The SV1000 is more narrow than the R1 - so it will fit in the stock location and will fit in a stock width Rhino without having to bring it back or widen the b-pillars.
My SV1000 was alot of fun.....but I just prefer the R1 sound and powerband over the v-twin. Either swap is a KICK IN THE PANTS, and makes ripping around in your Rhino waaaaay more fun!!
I, of course, run the Weller swap kit because it's extremely tough, very low maintenance and a pretty straightforward set-up. Changing gearing is as simple as changing sprocket sizes on front/rear and we just don't have any chain issues. No reverse hasn't been an issue for me.....I dont go out alone and I am used to parking/stopping without reverse anyway because I'm a quad rider, too......and have never had reverse on my quads!
The cool thing is, as consumer, we actually HAVE choices out there these days - and it's just a matter of figuring out which choices meet your particular needs the best.