These threads, unfortunately, always turn into an us vs. them or my sport is better than your sport conflict.
At my local club, I think there is SOME effort made to try and make the IDPA matches at least somewhat realistic and to provide some training for "real life". A couple examples: at least one match a years is shot in the dark with flashlights. I was at a match were the senario was given and the first shooter said, "I wouldn't even draw my gun, I am sitting in my car with it running, so I would floor it and leave the situation". They guy was credited for a max score for resolving the situation without firing a shot.
Bottom line, IMO and in my experience, IDPA does make some attempt to provide citizens who carry concealed with some training for real life. Note that I said, SOME TRAINING". This doesn't mean that shooting IDPA is a complete course, but it does teach you gun handling with practical guns, concealment, and at least here flashlight.
IPSC makes no attempt at all to be "practical" The guns used by some competitors are designed strictly for this one application (IPSC) and would never be carried concealed. The goal of most IPSC matches is to have a high round count, afterall, shooting is why we are there to begin with. Sound defensive shooting tactics have no place in IPSC: it is purely a game with no illusions of anything else.
I have enjoyed both, but IPSC certainly wins out as being more fun. Anytime you shoot more, it is going to be more fun. Either one will improve your gun handling skills greatly.