The cheapie Garmin units have really bad antennas, folks, so avoid those (mostly the eTrex units). Look for a model with a Quad.Helix antenna (the GPS-V, and other larger units, or any Magellan Meridian or Sportrak unit).

The antenna matters only if you want it to work well in urban canyons, treed areas, front seat (not dash) of your car, or inside a wood framed building.

After that, consider mapping software and detail for the areas you plan to use the unit -- and the cost of that software. Also consider how much storage the GPS has built-in (Garmins and Sportraks) for maps -- many of them are very limited and will require swapping maps in/out of memory over a slow serial link prior/after each trip you take to a new area. Units with expandable storage are preferable in this respect (Meridians).

Cheers