I have been x-country dirt biking, and snowmobiling with bar mounted GPS for a few years, here's some highlights of my experience.
My current GPS is a Magellan Exploris 510 handheld, and I love it!
I bar mount it, and have great success using lithium batts, or rechargables in the cold.
A great feature of the Magellan, is "active suspending". Basically, when you suspend the unit, it shuts down the screen but continues to locate and lay tracks with much less battery usage.
It also can be externally powered through an optional water resistant usb cable.
-I have used several different Garmin's and had lots of problems with the batteries momentarily disconnecting causing powerdown, and screen freeze. I would not recommend any mid to lower end Garmin that uses AA batteries.
-The Rhino units from Garmin, are certainly another breed and work very well, especially being able to see your riding partners on your map..... but I could NEVER convince any of my partners to buy such an expensive unit. This meant spending alot of money on features I do not use, not for me.
-Almost any GPS will work with Garmin software, so this doesn't need to be a deciding factor at all.
-Smartphone app GPS work great but depending on your phone battery can lead to trouble. Most GPS units, even those that use internal batts can accept AA, or AAA batts to keep you tracking when s*** hits the fan in the back-country.
I prefer hand held units that:
-can be bar mounted
-quickly removed to carry by hand or in pocket
-uses replaceable batts (quick change on the trail)
-has the capability of external 12v power supply
