Basically, tree cover issues is a major problem no matter whether you use a Garmin, Magellan, Cobra or Silva unit. The reason for this is that chlorophyll in the leaves on trees absorb the microwave frequency of the GPS signal. The GPS sats broadcast at two frequencies (L1=1575.42 MHz and L2=1227.6 MHz) with civvy units being able to pick up L1 and military units picking up both (allowing them to use dual freq error correction). The L2 signal is a little better in tree cover than the L1 but only marginally.
Let me give you some background ... the Garmin/Magellan thing is like the PC/Mac thing or Windows/Linux thing. If youare a new time buyer I wouldn't buy into that ...
Sales seem to indicate that Garmin units outsell Magellan about 9 to 1 and I don't think that this is an accident. Personally I've had several Garmin units and they have been great and the customer service (in Hampshire) is excellent. I've had units repaired free of charge that were well outside the warranty period. i can't report the same excellent level of product quality or support from Magellan.
So, beyond that, it's down to the antenna and loaded software. Some say that the antenna known as the quad helix (which Magellan primarily used but they are now shifting) is better under tree cover than the patch antenna of the eTrex. I have units using both types on antenna and I'm convinced that the apparent better signal lock of the Magellan is more to do with the software pretending that the lock is still in place when in fact it has been lost than an antenna performance issue. Experiments seems to show that this might be the case. You also need to hold the units right ... the patch antenna units (like eTrex) prefer being horizontal while the quad helix uniuts prefer to be held vertically for the best signal.
To get the best signal under a lot of trees I normally use a re-radiating antenna ... I put the pickup antenna high up on my backpack and keep the re-radiator antenna near the unit. You do need to carry a small 12v battery but it works and works well.
Basemaps aren't too bad but they aren't great either. You can splash out on extra maps but if you really are serious and want the best, I'd suggest getting a Pocket PC and hook up the GPS to it and getting Memory Map ... that way you get real time plotting on an actual OS map. This is by far the best combo and you can work with 1:250000 for driving and 1:50000 or 1:25000 for greater accuracy. This is the mutts whatisnames combo in sat nav and mapping when not in built-up areas!
