RZR pirate, I went with Racertech medium's and it is night and day difference. I have never regretted that choice, it is one of the best things you can do to your machine! Well worth the money spent, after the first ride I was very satisfied. There is a reason there are so many vendors providing spring sets for the S models.
From my research at the time, roughly a year ago, there were basically 3 choices I knew of at the time, Racertech(RT), Shock Therapy(ST) and Bandit were just on the scene. The Bandits were not dual rate on all four corners and I couldn't find much info so I removed them from my choice. But since then, many people on here have purchased Bandits and really enjoy them. If the budget is thin, Bandit is the way to go!
My riding style is mainly slow speed trail riding in the Ozarks. Lots of rocky, rough terrain and tight wooded areas. There is the occasional higher speed runs with some playing around. I do some back road runs and catch slight amounts of air on those. For reference, I have left my rear sway bar connected. Some people remove the sway bar, I may try that in the future.
The factory set up had several issues for me. 1. The front springs were way too soft. I had to keep the shocks turned up to 12 clicks or more just to keep it from being scary and unstable. This is why you don't want to just change the rear tender. 2. The rear dual springs had two main issues for me, first the tender spring were of such low quality that they had completely collapsed by 150 miles, secondly the rear main spring was way too stiff. So, the collapse tender caused a loss of about two or three inches and left me riding on an overly harsh main spring, which literally felt like a kidney punch over every little stone. The fronts had sagged some also, which explained all the bottoming out. The machine was still fairly new, so I was able to notice an extreme change from the first couple outings.
I installed the new springs and instantly knew the ride height was restored and maybe a little more. I had 14 inches in front and 13.5 inches inches in the rear with a 1/4 inch preload. The new ride is so much better than the original equipment it's really not a comparison. At low speeds, <15mph, it is so much smoother. No more kidney punches. It is not plush or squishy by any means, its firm but compliant. The ride at higher speeds, >30mph, is almost like a hover craft even over very rough terrain, remarkable. I can fly down open trails and even catch some air, which was almost impossible with the factory set up because the back end would kick up so bad, resulting in terrible nose dives off everything. All that is resolved. Also, I run about 2 clicks in the rear and 5 clicks in the front shocks, I like it soft. I do have to adjust it up some clicks if I am doing >30mph regularly. Also, there was another benefit that is hard to explain, the springs simply react better to the terrain and situations. I would suspect some serious testing went into the spring choices by RT.
There are many good choices out there, do a little research and I think you will find something that fits the bill. My thought at the time was, if I was already going to spend hundreds of dollars on new springs, I felt it worth spending a little more and getting dual springs all the way around. From what I have read on here, you would get very similar results from the Bandits, if budget dictates lower cost, then get a set of Bandits. You will be glad you did.
Hope this helps.