IDPA is more "real world" with practical guns and realistic scenarios, drawing from concealed carry, mandated use of cover, and short, self-defense "problems," often based on real-world events, that have to be solved. ("Real world" scenarios are an ideal, not always followed at some clubs.)
IPSC matches tend to be more of an athletic event, with lots of running, no use of cover, impractical (for the street) guns and holsters, and very high round-count stages that bear little or no relationship to anything you're likely to find in the real world. Recognizing the problems of bizarre equipment - which turns some people off - IPSC now has "stock" and "production" categories to allow shooters with more normal equipment to compete.
Maybe it's a matter of dedication or simply "trigger time" but the top-ranked IPSC shooters are very, VERY good. Once he knows the rules, an IPSC Grandmaster will dominate almost any IDPA club match.
To be kind to your enemy is to be cruel to yourself - Sun Tzu