I competed for several years exclusively in IDPA before crossing over to USPSA. Now I shoot hardly any IDPA.
Starting out, IDPA will help out with giving you practice drawing and reloading with a cover garment. You'll have to be at least minimally aware of using cover (though the way you use cover in IDPA is not the way you would use cover in real life). Because you don't get to walk through and air-gun the stages in IDPA, until you start realizing that all stages are made up of the same basic pieces, you will be shooting somewhat reactively, which is not a bad simulation of real life.
But here's the thing. After a while, you'll be plenty good enough handling a cover garment, your use of cover will essentially become just practiced footwork (“…foot goes here so I don't get a procedural error…”), and you will realize that every stage breaks down into two or three little mini segments that you have done plenty of time before. Also, since the 2010 rules updates, IDPA has more or less made it against the rules to shoot on the move, so you won't even be getting to practice one of the main reasons I originally got into action shooting.
While USPSA is definitely more game oriented, I believe firmly that shooting USPSA matches will enhance your gunhandling, your accuracy, and your shooting speed much faster that just about any other live fire available to people who don't have a SWAT shoot-house available to them in their free time. There is a distinct difference between developing a stage plan for a USPSA match (which involves planning footwork, deciding when to do reloads, and grouping targets) than there to developing a “stage plan” for a real life encounter (which involves applying whatever actual tactical training you have managed to take on your own), but once the plan enters your mind, the USPSA experience will help you execute it better.
As for equipment, odds are you can run your carry gear in Production division in USPSA, as long as you grab a few extra magazines and mag pouches. If you can't make it in Production (most external modifications are prohibited), you can run Limited or Limited-10, or even Single Stack if you are a 1911 shooter. Trust me, plenty of people run special equipment for IDPA; if that doesn't stop a lot of folks from just shooting what they have in IDPA for practice, there is no reason it should stop them from doing the exact same thing in USPSA where the quality of the trigger time is, in my opinion, significantly better.