As a scientist, I've used a lot of different GPS units.

The following is provided as a comparison between HANDHELD devices (if you're keeping it in your car, these are not what you're looking for).
I like Garmin over Magellan because Garmin GPS units are easily used with one hand.  Using a Magellan with one hand is like trying to wrap your fingers into a weird chord on a guitar, and therefore if you want more info on a particular Magellan, ask a guitarist.

Here's my breakdown of the *IMPORTANT differences between the Garmin Handheld units, including the max I think you should expect to pay for one on Ebay:

Garmin Etrex - $100.  Get this one if you want to keep it simple, don't have much money, want a starter to learn how to use a GPS, or are happy with paper maps but want a little extra security. Usually gets 24 hours out of batteries.

Garmin Etrex Camo - $120. Pay an extra $20 to get camo green instead of yellow.

Garmin Etrex Venture - $150 - keep the familiar features of the original Etrex, but add a little more accuracy and limitted map capabilities - I'd go with a Legend instead. Sometimes get 24 hours out of batteries.

Garmin Etrex Summit - $150 - add a REAL compass and altimiter that work even if you're not moving. 16hr battery

Garmin Etrex Legend - $150 - add a decent map and a few minor options to the Etrex for $50.  18 hour battery.

Garmin Etrex Vista - $200 add another $50 for a REAL compass and altimiter that works even if you're not moving.  12 hour battery

Garmin Rhino 110 to 530 - $250 to $500.  My personal favorites.  For about $150, put a two-way radio inside your GPS.  Well worth the price if you are ever afield with a partner, family, or a team.  The Rhino line starts with near Etrex capabilities with the 110-130 and they have a new one every time I look.  

All "C" Models - Add color for $50, but lose battery life (some use more batteries instead of losing battery life). 
All "x" Models - Add external memory capability (includes 68mb card) for another $100.

**Please note that now, a full year after the guide was originally written prices are becoming out of date, but the concepts are still the same.  There are some additional units that were released last summer that I haven't had a chance to investigate, I'll update when I get some free time.   

Now, GET LOST!    But don't forget to take your new GPS along so you can find your way back.