Over spring break, I have the opportunity to go to either Bryce or Zion. However, I'm a little conflicted as where to go. I would be looking to do mainly bouldering, and I have a crash pad as well as two spotters. Any advice or stories on either?
Neither place has much in the realm of bouldering, but Bryce (to my knowledge) has basically nothing climbable. Zion at least has about a dozen established boulders, most notably the globe near the entrance.
If climbing is part of your agenda in any form, Zion is vastly superior to Bryce.
If you want to boulder, consider making the extra drive to Saint George where a life times worth of boldering can be found with in about 20 minutes of the city.
Moes valley in saint george is where I camp for free when visiting Zion. A few hundred boulders of all grades thirty min from the main entrance of Zion and even closer to the western(?) entrance.
Boulder in Zion, there's a lot there...it's not really in any of the "modern" guidebooks or on the web, but there's lots of good stuff. Also the two aren't that far apart...go to both regardless!
I wouldn't call Zion a bouldering destination, but Bryce has nothing climbable in the park.
As others have said, Moe's Valley offers the best bouldering near to Zion. There are also some developed bouldering spots on the mesas near Zion and some more in the canyon. I'm not aware of any guide books for these newer spots, so I'd seek out a local to help locate the good stuff.
Also, Moe's Valley to the west entrance of Zion is at least an hour's drive. The camping is free and plentiful, just be sure to clean up after yourself.