Hi, Cliffy,
Not to beat a drum either way, but I've shot both and personally, I tend to prefer IDPA. I know other people prefer USPSA, so it can go either way. I usually wind up in a bit of general ribbing with my preferred instructor at the place where I take training. He's USPSA all the way. (well, that and he tends to shoot a lot of steel challenge with some old revolver dinosaur, Jerry Miculek or something)
Some people who didn't want to shoot race guns switched from USPSA to IDPA because they wanted to shoot God's own 1911. I think USPSA started the SS division to entice some of them back. I think this is a good thing in general because it's going to give people more opportunities to shoot and hopefully increase the base of the shooting population.
The reason I personally tend to prefer IDPA is I believe it more accurately reflects the sort of tactical situations I'd encounter in real life situations. I don't really compete to compete, but more or less in order to practice and get my brain working. There are some really goofy rules in IDPA, especially involving magazine changes, but there are goofy rules in all of them. I think some of these are just IDPA trying to make itself different than USPSA!
Anyway, back to the subject, in IDPA, you can't use extended magazines. 8 rounders are fine in CDP (Custom Defensive Pistol or .45 1911 division). I only compete against others shooting the same division, so we're all running around with pretty basic 1911s and 8 rounders. We're all required to use legitimate carry gear, or in the case of LEO, duty gear, which I think is a good thing for me, since I tend to use it as practice and to make sure my carry rig is working for me.
On the other hand, I've seen some USPSA shooters who get really good and can compete against the 10 rounders when they have 8s. Those guys are much better shooters than me, though!
All told, I'd like to see the SS division of USPSA grow. Make suggestions on the rules and maybe some day, I'll see you out there!
Cheers,
kk7sm
Note: I have written this from the perspective of an IDPA shooter because that's what I know best. I fully recommend and endorse shooting everything you can and figuring out what you like best, be it USPSA, Cowboy, IDPA, Steel Challenge, bullseye, GSSF, etc.