I've only ever shot a single USPSA match. Major difference is that there are rules and procedures to follow in IDPA for each stage. For the most part, people end up shooting each stage the same way. I personally don't really find air gunning to be necessary in IDPA, and it's not really allowed anyway. IDPA stages often dictate when, where, and how to reload, but not always. Bringing the speed of USPSA shooting into IDPA is definitely an advantage.