There are other shoots also, like Steel Challenge, ICORE, Bowling pin shoots, etc. Find whats close to you and shoot. You spend a lot of money for the guns and gear, shoot as many events as you can.
But for your question, IDPA is more restrictive in the gear like holsters, you have to have concealment clothing etc. In ISPC anything goes. If you shoot both, gear up for IDPA and you are legal for IPSC also.
Both started up to be training for defensive situations. To me they are just games and a lot of fun.