[TerryS]

Here's a rundown of what you should consider:

Frequency: Do you want dual, single or multiple frequencies?
Power: the more the wattage, the more the GPS will function
Resolution: The more, the merrier!
Colored vs B/W: Lol
Type of transducer.

When you know all these, it will now be a matter of weighing your options to see which GPS will work best for you.