The a magellan is ok, but just that, ok as it is a bit clunky and in need of an update, so overall the 510 is the go here.
So I pulled the trigger myself on a new 510 for Xmas, excellent form factor and very precise readings. Then on 6th ride, as I have it running on roadie only the screen froze. It would not let me save or discard the 6th ride. So I call my mates who all run Garmin to see if they have had this issue, a couple had and a reset fixed their problem, not mine however.
I spend an hour on the Garmin customer helpline and together we work through! Reset, Reboot, download latest firmware but screen still locked on and won't let if progress one way or the other.
Now I have to say I still like the Garmin form factor I have experienced up until the point it shit itself, but one offs happen, it just happened to be mine, but it is the frustration and dismissive lack of ownership from the retail bike shop in the Melbourne CBD (that's a hint) that has pissed me off, but that is another story on which an outcome is still pending but I have noted every detail of that exchange because I can't believe they just don't give a flying f*** at all.
But for your need I state that I would be happy to have a working unit back so would still say stump up for the 510 above the Magellan, it really is a nice unit.