You know, if used properly, I've noticed no difference in "spot-on-ness" between a Garmin and a Magellan. Garmins seem to get people closer to the hide location initially, but they still need to stand still and let them settle (and average) in the end, like a Magellan.
Just to complicate matters, I've used a Lowrance, too, and it seems to perform as well as the other two brands.
By the way, thanks for doing 15 minute averages for your hides. If I see a log saying "the coordinates are 10' off," I consider it a compliment regardless of the motive.
I've noticed that my Garmin eTrex has a nasty trick of assuming I've carried on moving in the same direction (kind of like momentum). It's particularly fond of doing this when it can see less satellites so often it will appear to track into a tunnel where there is no way it knew where it was. Once you know about this feature it's OK but for the fact that the built-in compass needs movement in order to get direction so I end up having to choose between an accurate location and a pointer which points in the right direction.
My eTrex does the same thing. I fix that problem with keeping a compass with me.