Another note --
Check to be sure your unit will accept an external antenna input!
Garmin sells an external antenna for about $70-$90. There is a similar unit available for about $25-$30 (I'll post the mfg later; it's out in the car). I've read specs that indicate the dB pickup exceeds the Garmin -- I can't vouch for that, but I can say it works extremely well and is about $60 cheaper!
It comes with a looong cord, incl magnetic and suction cup mounts. I have mine on the rear roof of my car, snaked around the back window, into the trunk, and up between the front seats.
We usually pick up 7-9 strong satellite signals, compared to 4-5 medium signals w/o the antenna. I have seen up to 10-11 at once! They work so well we've been meaning to buy two more -- one for wife's car, and an extra so we don't have to keep removing and reinstalling them.
(e.g. While hiking, place it on your head, under baseball cap; run cable down your back, down your sleeve to hand-held unit. Now you can put unit in your pocket w/o losing signal. [Be prepared for some minor adjustments when you forget about it and take off your hat to scratch your head; and try not to do this when other people are looking . . . ])
Batteries:
The GPS12 used 4 AA, was relatively heavy, and ATE batteries (6-8 hours?). So we added a 12-V power cord in the car. A hassle! Map76 is same size but takes TWO AA, which last 12-20 hours, and provides a much lighter package. Now we don't bother with 12-V cable. I imagine most of the newer units are similarly energy efficient, but can't say for sure.
Memory cards:
Can't say for sure, but until recently these were not available in the waterproof units. Seems like this should be possible. If available I'd look for that feature (but would NOT give up the sealed case to get it!). It would be great to have several cards with appropriate data installed. e.g. one for car, another for boat, one for hiking.
Ed