I shoot USPSA (still meaning to try some IDPA matches for fun)
If all you are after is having fun I think either would be a blast. (matches are just loads of fun IMO)
I chose USPSA to start because the rules seemed a bit simpler.
Down load the rule and do a quick read through of all of them, then and go shoot one!
I shoot Single Stack (1911s only, and I am in a 10 rnd mag state anyway) so I get to do lots of mag changes.
For USPSA if you are shooting Production (limited to 10 rnds in the mag-most striker fires and DA/SA pistols fit here) you will need at least 4 mags, 5 or 6 would be better and mag holders for them.
If you want to load to mag cap (assuming you can have more than 10) limited will let you.
IDPA if I remember correctly limits you to one in the gun and 2 (maybe 3 depending on cap) so you won't need as many mags.
(and you won't shoot as much ammo, shooting more ammo is good IMO, I believe this so strongly I miss targets just so I can shoot extra rounds and practice mag changes
)
You can go watch a match at first, but I would suggest taking you equipment and going ahead and shooting it.
There is a first time for everything
, might as well get the first one under your belt, much more fun to shoot than to watch.
At least in my club everyone is very helpful to new shooters, we were all one at some point in time.
Shoot+enjoy.