I too have a Magellan 315/320. It's been a reliable unit and although I'm new to geocaching, it's never failed in many hikes, drives, and sailing races. It's been abused in direct sunlight on the boat, and worked well down to -30C. That said, it's also had the firmware updated (download from Magellan). The major change in the firmware was lowering the speed at which averaging occurs, to less than 2m/s.
This helps greatly when sailing in light winds or geocaching. In comparison with a garmin 12XL on the boat (side by side on the bulk-head), the 320 regularly sees 1 to 2 more sats than the Garmin, and often has less error estimate by 5-10 ft.
My next will probably be one that can accept an external antenna, and that has the ablity to upload maps. A magnetic compass would be a nice addition, but I'll still carry my Silva.
There is a lot that can effect reception, and to say one is better than the other often is a matter of a few percent difference. If you are comfortable with operating your 315, stick with it till something on a new unit tips the scale. When you are seriously looking, seek out other cachers and tag along to "test drive" the newer units. Spec sheets never tell all about use in the real world.