I went cheap, and bought a Navman 210 a couple of years ago, for $99, from PlanetGPS. Since the changeover to "Northstar", I understand prices have gone up.
If you don't mind 3 minutes with a calculator, it's great.
But if you're willing to pay an additional $250 to have the machine do the calculations, AND you don't mind having to have a square box, instead of a neat 2 1/8" flush mount in your console of dash, go ahead and get the 310.
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Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury