There are alot of differences between the two sports that are little more than nuances, but add up in the end.
Some of the bigger ones that I would say are:
1) Speed - goes to IPSC, but I think that alot of this is optics. There is more opportunity to run in IPSC for sure, and the good open guys sure burn up the powder on a stationary stage. Lots of little stuff adds to it (sequence, cover, equipment, etc.)
2) Scoring - Ultimately, lack of consistancy and points down hurt you more in IDPA, as everything culminates with an aggregate time taken for the match.
Generally, little stuff like failure to neutralize (not so little), a few mikes and/or hitting no shoots can destroy your agg. time.
Scoring is weighted differently with short, medium and long coursed in IPSC, meaning you can sewer a stage (and I mean hard) and still do ok in a match.
3) Accuracy - is subjective to these two sports as the scoring is different, but overall, both have rules that allow for
some shooting to be far more difficult in IPSC; I'm not talking about the carnival stuff like texas shooting stars or the like - just plain marksmanship.
As I play both, I'd highly recommend each sport to folks in either one to try the other. If you have a safe full of pistols, this is the perfect way to rotate through them
