This may be a bit of a long answer to a short question. To let you know where I'm coming from, I am both a USPSA CRO and an IDPA match director. The short answer is that either can serve your purpose if you find a club that is legit and follows the actual rules. There are unfortunately some clubs that use the name but not the rules.
With the addition of Production Division to USPSA, some say in response to IDPA;s success, USPSA provides a division for stock handguns to compete against each other in USPSA matches. The approved guns for USPSA Production include your gun, and the rules can be found here:
USPSA Member Discounts and Promotions - United States Practical Shooting Association
In general, you will need a strong-side holster that covers the trigger, 5 magazines, aand holders for 4 magazines. Yes, you go onto a USPSA stage with a box of ammunition on you, and some stages require 32 rounds (more if you miss). You will shoot the same stages as the other Divisions, including the Open race guns, but compete in your Production Division against similar equipment. You may run across the rare individual who belittles Production Division, but in general competitors are supportive of newcomers. USPSA is the classic "run and gun" with emphasis on speed and pure shooting without much regard to tactics.
In IDPA, you fit right into Stock Service Pistol Division, and the equipment is similar except you only need 3 magazines. IDPA stages are max 18 rounds required. IDPA policy states that its target population for recruiting is shooters who have not competed before and wish to shoot essentially stock firearms.
IDPA is more oriented to shooting accurately from cover and employing "defensive" scenarios, and the scoring favors accuracy over raw speed.
The rules can be found here:
http://www.idpa.com/Documents/IDPARuleBook2005.pdf
There are some popular misconceptions about the two that deserve comment.
1. "IDPA has more rules" USPSA has 100 pages of rules, IDPA 82 pages.
2. "Using cover in IDPA slows you down." True. Run & gun in the open is faster.
It really comes down to the availability of a good club in your area.