We have been to Zion and will take a day trip over there from Kanab this summer.
Unless you are into serious hiking the best part of Zion is the drive down from the tunnel on Hwy 9. Otherwise it is pretty much a Park bus ride getting a sore neck looking up and the canyon walls.
We were at Bryce last year for 6 days and will be there 6 days again this year on our way to the Oregon Coast....that might be a little long based on the short time span between visits. But it is a great place and other things around it as well.
At Bryce you are looking down at spectacular beauty; particularly at sunrise. And, as said, getting below the rim is amazing down there hiking.
Enjoy - be safe.