Best bet, is a GPS 315, with the latest firmware upgrade(vers. 3.15), which is a Good geocaching unit.
Other GPS units have more of an other purpose:
etrex's - Good for Running, Jogging, or hiking.
GPS 76/Map76 - Good for Boating.
Meridian units - Good for all but Running.
GPS V - Good for all, but pricey.
etc
If you wanted to track yourself on an exercise program, get the eTrex.
For all around basic use get the low end Meridian Green.
For use in unknown areas, get a mapping unit.
So for just Geocaching, get the GPS 315, for all around economy, get the low end meridian, or an eTrex.