I'll try and help out a little here, first off for full disclosure i am an "idpa" guy. I am 32 now, started shooting IDPA the day i turned 21 and bought a gun. Took a few years off in the middle but IDPA. Recently I have been investing more and more time into looking into USPSA and have shot a few matchs.
IDPA:
IDPA prides itself on trying to be "realistic", problem is they sometimes try to hard and force you for the sake of being "realistic" into situations where you aren't being realistic. IDPA started when a group of guys split off from IPSC and started their own things. In the early years I beleive rules were made simply so they could be different from IPSC. No reason just that they felt they needed to do the opposite of what IPSC was doing. A good example is the not reloading while on the move, unless you are behing cover. If I am ever in a gun fight, if my gun is empty, I don't care what i am doing or where I am, i plan on reloading. The other is the limited mag capacity to 10 rounds. They took it a step farther recently, buy coming out with 3 guns that limit your mag capacity to 10 rounds in your AR!! :screwy: :shock: I don't know about you, but all my mags for my ar are loaded with a lot more than 10 rounds. The other thing that has gotten on my nerves lately about IDPA is their obsession with strange and awkward shooting positions, using strong or weak hand only. I understand throwing in strong hand/weak hand stuff, but it's getting to the point where there is just to much of it, in my opinion..
USPSA:
Less rules, but more race guns. USPSA allows you to shoot basically how you want, as long as you stay inside the designated scoring area. Can reload anywhere, shoot from anywhere. Some of the major/minor stuff can get overwhelming, and the open division just isn't for me. The 3 gun is actually established, with major matches everwhere that book up seemingly instantly. I still can't find a 3 gun match for IDPA... USPSA is more game orientated, where IDPA is game orientated but they don't advertise it as such. USPSA also has the steel challenge as part of it too. Which is also a lot of fun.
I've been an IDPA guy for almost 12 years, but this year I am switching over to USPA, the stages are more fun, and there is less ticky tack rules, you just go and shoot.. Plus the 3 gun is really a draw for me as well.