I'm fairly new at this and don't have much experience with GPSr's, but I recently got a Lowrance iFinder H2O and the thing that I really like is that it uses removable memory cards for its maps, allowing you virtually unlimited memory capacity.
The MapCreate6 software (optional) is also awesome. You can build custom maps by dragging your cursor around areas on a map on your PC and hitting a "Generate Map" button which builds a custom data file containing just the areas you've selected. Put the file on the memory card, put the memory card in your GPSr, and you're good to go! The MapCreate6 software also allows you to easily build a "corridor" map along a travel route. So, for example, I could easily outline my local area, then a "corridor" along hundreds of miles of highway, then my destination area (all with just a few mouse clicks), and fit that custom map data on a memory card easily. Very flexible!
I went with Lowrance because I've got a Lowrance unit for my boat, and I wanted to stick with the same brand, but now that I've got it I really like it.