I got to experience both for the first time this year.
I like doing both, but if I had to choose one, it would be IDPA. Their rules and equipment limitations are kind of an equalizer. You don't always get to shoot from the most comfortable position or reload when it is most convenient in IDPA. Targets sequence is also much more important/regulated. I have talked to USPSA shooters that really love the ability to push their shooting talent without having to worry about procedural errors or rule violations assigning penalties. Accuracy is the primary test in both. The ability to have the shortest split time between shoots and quickest target transitions is also paramount in both competitions. Shooting positions is one of the major differences. Shooting while making sure that 100% of your lower body and 50% of your upper body is behind cover is IDPA. Or, just standing in your favorite shooting stance at a location that gives you access to the most targets before you have to move is USPSA.
Other little things like airgunning and the number of magazines you can have are just a couple of the IDPA rules that make people migrate to USPSA.
They are both fun and challenging...Plus IDPA has Defense Multi-Gun where the fun is really intense.
Just my .02
Bp