These threads usually turn into bashing of one sport or another and I usually stay out of them.
I shoot both sports and have invested the time and energy to become a NROI certified RO and an IDPA certified Safety Officer and Safety Officer Instructor. I shoot both sports poorly but I have a good time. In USPSA I will never become classified in the Open Division because it does not serve my purposes. I marvel at those that do it and do it well but it is not for me.
IDPA was the brain child of not just Bill Wilson but of a group. Bill Wilson stepped up and put up a big chunk of change to get it going. Smith & Wesson was right there too but to a lesser degree.
Bill Wilson was an accomplished IPSC shooter but felt that the "P" in USPSA was becoming a little foggy so he decided to put up instead of shut up. I think some of what he wrote was a little bit of bashing, but he paid for it so he can say what he wants I guess.
IDPA is a privately owned corporation. When you join you agree to go by their rules. If you don't like it, you have the option of not shooting and not renewing your membership.
USPSA is more of a member owned company where you have a limited say through your elected area rep. If you don't like how he votes, you don't vote for him or you can even run against him. We are very lucky in Area 7 to have a very good rep in Rob. (Thanks Rob) All areas seem to have good representation by hard working people.
Since IDPA was started from USPSA, there are similarities but since the founding group was dissatisfied with USPSA, there are also differences.
USPSA has higher round counts, more open shooting, and is a much faster paced sport. IDPA is a limited round count type sport which is geared to every day carry type equipment. Some look at it as "Real World" I am not that naive. It is not real world unless the targets shoot back. USPSA saw a loss of revenue from those shooting IDPA in the CDP and SSP divisions so they filled the need by developing the Production division and the Single Stack division. Smart move as far as I am concerned and a good way to let everyone shoot both sports.
The best way top decide which or both are for you is to play the games. Not just one time, but three or four each. The match fee for USPSA includes a stipend to headquarters so you are contributing to USPSA by shooting the match. IDPA requires membership after one match because there is no other funds sent by the shooter. With either sport you are paying. IDPA wants their money up front (It is that private business idea again)
Either way you look at it, you get to spend time with a great bunch of like minded people doing what you all enjoy. I have made friends from all over the country playing these games and I have never regretted any time on the range even with my poor scores
Edit to add: If anyone would like to try either sport in the Production division or the revolver division (SSP ESR or SSR in IDPA) I will make this offer. If you are experienced enough to attend a match here on Cape Cod and don't think you have the right gear, I will let you use mine for a match. If you have never tried either and you are not sure you dare, I will give you a hand with a little familiarity to the sport before your first match. Only thing I will not do is explain USPSA scoring it still confuses me