If you have an RTX-SE Viper, do you have the Float 3's? I think that the standard coil-over shocks are much better for a trail sled than the Floats. I had the Viper LTX-DX until it was totaled in an accident by a family member. My buddy had the LTX-LE with the QS3 adjustable shocks, and except for the adjustability, I think that my DX rode just as well as his LE. Adjustment is a nice feature, but for 90% of my riding, it doesn't make a difference. After my Viper was totaled, we both upgraded to the Sidewinders. I got another LTX-DX and he got the LTX-LE, again, he has the adjustable (on the fly) shocks, and I have the standard coil-overs. I think the LTX is the perfect length for a long distance trail sled; we put in a lot of 200+ mile days, and it is a very comfortable sled.
My stepson has an RTX-SE Sidewinder, and the Sidewinders SE's come with coil-over shocks. If I was doing shorter trips, or if I was always riding on tight twisty trails, the RTX might be a better option, but I think that the LTX handles much better in more varied conditions. When there is fresh snow, loose snow, it is less likely to get "squirrely" in the rear end. I just rode it on Sunday with about 8-10" of fresh snow on the trails, and it was great. I also think that the 137" length bridges the stutter bumps better than the 129", but others might disagree.
If you get the Sidewinder, don't let your wife ride it, or you will never get it back.