I have shot a lot of IDPA and a bit of USPSA. I like both, but I would say that I probably prefer IDPA. IDPA is less about the gear and more about the scenario, but some complain about the 'rules'. I think it is easier for an IDPA shooter to transition to USPSA since it is less restrictive than the other way around. I think the average USPSA shooter probably shoots at a higher level of technical competency than the average IDPA shooter, of course these are broad generalizations.

I was a match director for IDPA for a number of years and I tried to design courses that followed the intent of IDPA while offering an adequate round count and free form solutions. That being said, the typical USPSA match will offer a higher round count per stage and match.

One reason I like to shoot IDPA is because it is scenario driven. A good stage will have a brief setup indicating how you got yourself into this mess. A good USPSA stage defines the fault lines and shots on target without any pretense of 'reality'. This is not to say that you cannot shoot a USPSA match using the same tactics as you do an IDPA match. You won't come in at the top, but who really cares? Coming from CAS, you might like the stage descriptions in a good IDPA match.

The bottom line is they are both fun and you should explore both. Quite often your local club will do a good job in one and not the other. Pick one that is the most fun and run with it.