I've done both --
If you have the time, do the
Festival. It's the same roads and mostly the same ride(s), spread out over a couple days. If anything, you get to see more of Moab that way. The Century doesn't go into Arches nor to Deadhorse, and if you head there, you can't miss those.
The weather can be iffy (we got caught in an afternoon snow flurry one day. Hey, it's
March. It can happen.), but it's awesometown compared to later in the summer. There's a reason one is in March and the other in late Sept.
The organization is great, and Moab is a small town. Food is good. Margaritas are good. Coffee is good (
Moab Garage Co, I'm looking at you).
Non-riding options? well, it is like
Mountain Bike Mecca, or something. So I've heard. There's also hiking in Arches etc. There's rafting, but not sure how much in March.
Too far to include Grand Canyon, at least for a week trip it'd be a stretch.