DX3R More options, Larger memory bank, Telemetry and DSM2 Is faster
DX3S Less options but already alot for a new person to get overwhelemed with. DSM1 is slower but as a Casual Driver you wont feel a damn difference.
I barely felt any difference form Spektrum to Spektrum Pro. Maybe a little smoother but i am not sure if its the 2.4 or the servo. But i think the S has Telemetry also ? As a racer i bet you can feel the difference but as a casual driver or basher i doubt it. Heck i see some of the guys going to IFMAR with still FMs and reg DSM and 2.4 Kos
AM to FM to 2.4 will feel difference though.