Snoflewis is right...you need to demo them...no one's comments are going to be as useful to you as your own!!
Between the RDX Mid and RDS Mid:
I demo'd the RDS 001 Mid....it actually didn't feel as different from my RDX Mid as I expected it to based on the stats.
The additional stiffness of the RDS wasn't as "bad" as I thought it would be, and the RDS did swing heavier, but not so much that I was late on shots over a set and a half or so using it. RDS did provide more power, which most might appreciate. I was able to switch between the two during a match with a friend and I didn't feel like I had to make a big adjustment. I still prefer the wet noodle feel (as my buddy refers to it) of my RDX, but could use the RDS as well.
Between the RDX/RDS Mid and RDS MP:
I hit w/the RDS MP again yesterday (one set, went to tiebreaker, so 12 games). A nice racquet, though I had to adjust to the additional power...it has more than the RDX MP. Initially I was hitting a lot of service returns long on both wings w/the MP, and had some problems w/my forehand (which is relatively flat most of the time) as well. If I was going to buy this racquet I'd likely string it tighter than the demo (which was at mid-point tension) to tone down the power a bit. I would definitely inlude the RDS MP in a demo list over the RDX MP.
The RDS MP has a good solid feel like the RDX/RDS, more power and larger sweet spot. Volleys very easily, and is still very maneuverable in spite of the larger head. If I wanted/needed the extra power that the RDS MP offers, I would be happy to use it.
I wasn't playing a heavy hitter yesterday (more of an angles/spin guy) so I didn't get a good feel as to how it would fare up against a heavier ball. Felt solid on his hardest shots, but he's not a boomer...
Stop asking, and go demo.
(I should note that my first reply above was regarding RDX Mid vs. RDX MP.)