A great resource for places to shot in southern UT is the book "Photographing the Southwest, Vol.1 - Southern Utah" by Laurent Martres. Lists a lot of great locations and directions on how to find them and of course tips on how to get the best shots. I found the book to be very useful, even for a hack like me.
With 4 or 5 days between these two parks you can actually cover a fair about of ground. Bryce is the smaller park and could be covered with a one-night stay so that you have at least one shot at sunrise and sunset. You can get nice shots from the rim of the main amphitheater or you can hike below the rim a bit to get a different perspective.
Zion takes more effort to get to the better locations. It is a beautiful park, but honestly I like other area of southern UT better, so I'm not that well versed with where to shoot in Zion. But after seeing Geonerd's photos (great shots!), I will have to go back. The Narrows has been on my list for a while, but I just have not been back to the park.
If you can get 5 days, you might also want to think abut exploring along scenic highway 12 between Bryce and Capitol Reef. There are some great trails and wonderful scenery all along the route, particularly around Escalante. Devils Garden is easy to get to along the Hole in the Rock Road and further on that road you can try Spooky and Peek-a-boo slot canyons (there are other pretty slots in that area, but I have not visited those).