A local shop has two used mountain bikes that caught my eye. A 2003 (from what I've researched) Trek 4300 Alpha and an unknown year Gary Fisher Marlin (Gold Series). The Marlin (priced at $220) has disk brakes but the back one seem like the back wheel doesn't have a lot of stopping power. The Trek 4300 (priced at $200)won't shift and is a little beat up. Are either of these bikes worth buying and investing a little money in to get me out on the trails?