In addition to these choice you will need to decide whether to place the tracklog in "automatic" recording mode or "time" recording mode. In automatic mode the unit itself decides when to drop a bread crumb (trackpoint). (The G-III family also supports a "distance" recording mode.) Generally, in automatic mode, it will enter a trackpoint when you have turned more than 25 meters (82 feet) from a straight line projection from you last point or you have significantly changed the speed from the last entry. Using these two criteria allows the Garmin to accurately map your journey, however it becomes difficult to judge exactly how much data can be collected before the tracklog becomes full. Some units will also make a log entry when the unit draws a new screen. With a typical 1000 point log you could overflow the log in 40 miles or in 400 miles depending on the terrain and your driving/hiking/riding habits. On the G-III family you can change the setting for the turn distance. The Street Pilot uses 50 meters by default and this turns out to be a good setting for driving down the road. This, of course, will increase the length of data that can be collected at the expense of accuracy on turns. The etrex vista, legend, and venture have both time and distance. Automatic mode has a setting where you can adjust the sensitivity to distance from the projected straight line from less to more often.

UPDATE: Looks like the eTrex Legend is the way to go. Thanks to all who mailed!