go the smartphone route with a gps app. your gonna have a phone with you anyway, so having a separate gps unit just adds more stuff to your pack.
get an android phone like verizons droid razor maxx (maxx model has the bigger battery and it will last for your ride). i use the my tracks app from the google play store. the my tracks app is created by google employees on their spare time and its easy to use. i love the app. after you record your ride you can send it to your google maps online and check out your ride and stats. i use the my tracks app when i hike, road bike, mt bike, kayak and atv…works every time. i keep my phone on the top pocket of my camelbak or clipped to my side when i road bike with my music playing from the phone too.
worst case scenario with the smartphone battery life is to get a small portable charger if you see the phone is getting low on battery. i know its adding more stuff to your pack but they have portable chargers that weigh a couple ounces and are compact.
i also use the all trails app to find hiking trails close to me using the gps. i wonder if anyone knows if there is a mountain bike app that offers this.