USPSA and IDPA have some things in common, but they emphasize different skills. You would be penalized for shooting USPSA style in IDPA, but not the other way around. Shooting IDPA style in USPSA would just slow you down and confuse observers. heheh IDPA does require or encourage annual membership.
BTW, GSSF requires membership to participate, but it gets you an annual discount on a Glock pistol, and chances to randomly win pistols throughout the year. (My wife won one.) There are different formats, indoor and outdoor. The outdoor Glock Challenge also offers chances to win cash prizes. The one my wife goes to routinely, indoor Glock League, the hardest part is shooting within a vaguely defined 8" circle at 25 yards, 10 shots in 15 seconds.
Steel Challenge goes way beyond the basic constrained range experience. You might be surprised. Hitting the plates confirms minimum accuracy, but the challenge is to do the target TRANSITIONS fast while your dot is bouncing around. You learn to regulate your speed, use your vision differently, use your ears, call your shots, maintain body alignment during transitions, and not over-confirm your aim. The targets are in different arrangements in each stage, which might not seem that challenging...until you try to run the stage FAST. That's when you might find that you can in fact miss the huge, rectangular plate!
Anything that tests your skills and puts pressure on you is beneficial. Just get started.
USPSA shooter doing Steel Challenge for the first time. Shows all the stages, although sometimes one plate is out of the frame. (Another benefit is practicing the draw 5 times per stage!)