IPSC because it's what is available. I just started about a month ago. It is very addictive. I've missed the last couple of Tuesdays because of work travel. I shot my first full match Saturday. The range I shot at had 2 texas stars. I nailed my second star with one shot a plate and that was a real ego booster. I totally screwed up my qualifier stage and that is humbling. I like the idea of IDPA but there is nowhere local to shoot it.
Things IPSC is making me improve:
1. Grip, bad habits you get away with just shooting paper don't fly.
2. Magazine Changes, I don't believe in the Speed reload for deffensive purposes, however the mag changes do help build muscle memory.
3. It reinforces the necessity of good fundamentals. When I try to move to quick and don't focus on sight picture, grip, trigger squeeze, etc..., I do very poorly. When I take my time I do far better.