I have a DX3R - Brilliant. Features are top drawer. Ergonomic, easy to use, only 4 AA batts, so light weight, perfect spring tension in the wheel.
The DX3S has the Telemetry - Do you need it?
DX3R has lower latency with the DSM2, but I am not so sure 99% of the population can tell the difference between 11 and 5ms latency anyway
The DX3R is a really nice peice of kit and the new DSM2 receiver modules are lower priced than the DSM. Just bought a new SR3520. Used it this weekend, and whilst I doubt it made me perform any better, it didn't hinder my best result yet on it's first outing.
SR3520 DSM2 - £46.25 from Microtech
SR3500 DSM - £60.38 from Microtech
Finally, the DX3R is backward compatible, so you can use all the old rx without problems.
Hope it helps