I like the HRM and altimeter function for mountaineering. Mr Mud has the right idea with setting your upper heart rate. It's great because you can set your pace and know when to take a rest.
I use the Sunnto X3 (?) it's $150 and pretty light. I don't know how light the Garmin would feel on your wrist.
The HRM would allow you to do threshold training which in time helps you areobic threshold (?? it's been a while). Essentially it lets you run at a higher HR, think Lance.
But man, a HRM & GPS in one is pretty techie.