I have a Magellan Meridian for marking rocks, the reason I chose it was because it was suppose to have a better antenna in it than some of the others.  The other reason was because the button for marking is on the front.  It makes picking rocks fun (almost) it will guide you in the most efficient pattern from rock to rock when picking them.

I will add the Meridian would be an older obsolete model now, but it did work perfect for the job