I shoot SR9 in IDPA. I'm not sure how competitive the SR9 would be in Production USPSA/IPSC.
USPSA tends to have a higher round count and is more pure "run-n-gun". IDPA was created by a few of the IPSC founders with the goal of being more self-defense oriented. Most of the stages are of the "I'm minding my own business doing such-and-such when...". There are restrictions on the type of holster and mag pouches you can use, and limitations on the guns themselves.
There's a whole mess of argument on whether or not IDPA is good for defensive training or not, and I'm not even gonna touch that. But if you want to shoot more like you would in a defensive situation, then I'd suggest IDPA, where use of cover is required, as long as other considerations to try to simulate defensive problems. If you just want to run-n-gun, USPSA/IPSC, though I know loads of folks who shoot both. I would, but that's too much ammo for me to buy.
-JT