Generally, USPSA is more "freestyle" and has a higher round count than IDPA. The scoring's different. too.
In IDPA, the targets are generally are shot in a prescribed order, and you have to shoot and reload from cover when it's available. IDPA stages also generally involve fewer rounds than IDPA, and you have to wear a concealment garment. Use your Utube foo to watch some IDPA and USPSA videos and to get a general feel for each.
Some like the freestyle nature of USPSA, whereas some like IDPA because of its emphasis on concealment & cover. Keep in mind, though, that both are games, not self-defense training.
As far as gear, definitely read the rulebook, but kydex rules. A OWB kydex holster that covers the trigger guard and kydex mag holders are a good start.
What gun would you plan on using? That might help push you into one or the other game.