my brother has a trek 7.2 fx. we're about similar fitness but i'm trying to get in better shape to improve the "engine". with which mtn bike am i likely to have a better chance at keeping up with the lighter, 700c x 35 hybrid, the 29er or the 26? we ride poorly paved trails that are hilly. anyone dare to guess a percentage difference in speed? i'll give up 20% speed for the more comfort / ability to handle poor roads. currently checking out the trek 6000 (26") and the gary fisher cobia (29er). thanks!