Anything considered a "mapping" unit should work just fine for you. By mapping, I mean something that you can download topo maps onto (etrex Legend or better, Rino 120 or better, 60 series, 76 series). This does not include the basic etrex, Rino 110, Forerunner, Foretrex, or the new Edge models. The models with the quad helix antenna, like mentioned earlier, will get slightly better reception. The Rino series also has this antenna. The Rino series units also have a radio, which is nice if you are working with other folks separated in the field and you need to communicate, otherwise it's just extra stuff to haul around.
The ability to plug in an external antenna for improved reception/accuracy is a good option, and can be found on the 60 series. Other units require a rebroadcasting antenna with its own power supply, which is not widely available. I do lots of trailbuilding, as well. I'm currently using a Rino 120, but find I don't use the radio as much as I thought I would at first, and would like the color screen, electronic compass, and more memory for storing maps, waypoints, and tracks. I will eventually upgrade to the Garmin 60CSx when it's finally released.