I'm a big fan of Garmin GPS's. Currently I'm on my fourth GPS (in about 8 years). I started with the GPS 12, then went to the eTrex Legend, then GPSMap 76S, and now the GPSMap 76CS. Here's some pearls of wisdom I've learned from this...
When buying a GPS, don't forget to budget for the maps which are usually an extra $100-150. There are several non-mapping GPS's still on the market, don't bother with them (or I can sell you my old GPS 12). Spend the money and get a mapping GPS with the maps.
I haven't use Magellan GPS's so I can't say how good they are, but... Garmin has roughly 80% of the handheld GPS market. Magellan has maybe 18%.
Having a barometric altimeter and magnetic compass isn't as useful as you might think. Oddly enough, the only time I use the magnetic compass is when I'm in my inflatable pontoon boat.
The more memory you have for maps the better. The 8 meg that was in my eTrex Legend is simply not enough today. The GPSMap 76S has 24 meg and that's just barely enough for short trips. The 76CS has 115 meg and that's great! I believe that the newer Garmin units all have 56 meg or more.
Magellan can use flash cards to expand their storage, which is great, but their map management is terrible so you get some bad with good.
Well, I could go on and on about GPS's so I think I should stop here. If anyone has specific questions about them I'll chime in again...