Oh wow the classic gps question...
I might not be the best to answer that since I am an over the top navigational nut

I run a laptop with oziexplorer for topo & aerial photos and Iguidance for street nav...
For a great setup I have seen the Garmin 276c used by lots of folks and it does streets and topo pretty well, also removable for theft issues.
The Lowrance Baja big sucker is something I would have if not for my laptop, best screen, super tough but not as easy to remove.
I just got a etrex legend color at the last REI garage sale to play with and it seems pretty good, small and I need topo maps but pretty good.
The real questions are what are your plans...
Streets & off road?
USA only or international
are you good with a map & compass?
do you want to be able to do other things also (like I want to journal, kids watch dvds, internet access, download pics and more,,,,hence the laptop)
is your rig open air & dusty, is water an issue, theft?
All in all lots of people seem happy with the garmin 276c and I was just looking at it last night with a dr buddy at compuplus.com I think for $400...not sure if you still need maps or not with that.
REI has lots of stuff and other outdoor shops.
On the trail length none of the trails in sedona are long...couple of hours each really
I am sure others will chime in with gps suggestion also. lots of threads on the main mud forums