I use the Garmin rino 120. It's also a two way radio. You might not need the radio function but, if you know anyone else with a Motorola talk-about or similar you can communcate with them as well. The 120 already has an external GPS antenna. I hardly ever lose signal except in the most dense leaf coverage and that usually results in less accuracy rather than losing the signal totally. I have downloaded the Topo maps from Garmin and they are very detailed and useful. Also, if you know of another person using the rino 120 you can send your location to them and you show up on their screen as a waypoint, how far away you are, the direction and how to get to them. Pretty cool! Not that it's really necessary to do so when Geocaching but it is useful when hunting. Anyhow.....Good Luck.
BTW: This was an upgrade from the yellow etrex. HUGE difference!
This is no time for fun and games...FIND THE CACHE!!