First I'll admit I'm biased. I played IPSC first.
Personally I don't shoot many IPSC pistol only matches anymore, it's strickly 3-gun for me. Afterall why leave the rifle and shotgun home? IPSC is fast, challenging, has a lost of movement and requires a shooter to think on his feet.
IDPA on the other hand is more real world (as much as a game can be). Although I'm certain with my IPSC background if I get into a firefight and change mags - mag retention is out the door. IDPA seems very restrictive to me, but some people like the structure and the rules that penalize you for not using cover.
IPSC pistol stage - 22 to 36 rounds, typically a field course with multiple engagements and fast movement.
IDPA pistol stage - 6 to 18 rounds, typically 1 or 2 designated shooting positions reloading in total cover, shooting while using cover, requires retaining any mag with rounds still in it.
I like to run and gun, and espcially enjoy the challenge offered by the rifle and shotgun stages in a good 3-gun match.
Ryan