Some feature considerations when purchasing a GPS:
A basemap is highly recommended. The additional cost is marginal but it will increase the equipment's usefulness.
Look for a receiver with at least 12 parallel channels. This should include most units manufactured after 1997. Any older, single-channel receivers will be slower and not as accurate.
An external antenna jack allows the equipment to be mounted inside a vehicle. Good satellite coverage through a front windshield is not always possible. Riders can benefit from the ability to safely store the GPS inside a saddle pack with the antenna attached to a strap.
Computer interface is important when using the receiver with a computer. Be sure that it includes both data in/out ports, and make sure it's NMEA compatible.
The more memory, the better. Memory is used to load topographic or street maps into the receiver. Topo maps will use up a great deal of memory. Capacity should be at least 8 megabytes, or your GPS receiver should include the ability to accept memory cards to increase the memory capabilities.
Buy a unit that can use a cigarette lighter power cable whenever possible. If your unit only uses batteries, make sure to carry spares and use a battery solar charger for extended field use.
A rocker keypad makes the GPS much easier to use.
For ease of operation, find a receiver with the largest screen that can be realistically carried. Screen size is measured diagonally just like television sets and computer monitors. Color is great and helps define map features, but it will burn up more battery power.
Sooner or later the GPS will get rained on or dunked. Get a GPS that is water-resistant. Plastic bags and boxes can give electronics additional protection.
A safety cord tied to your GPS and to a part of your self or your saddle is also a good idea.
Check out the newer Garmin NuVi units for the best of both worlds. Waypoints can be entered as well as road navigation to get you to the trail head. The small size lets you slip the unit from your truck to your saddlebag.