05 Epic Comp vs 05 Racer X
I rode and 05 Epic Comp this past season and really liked the bike. It took some getting used to, but once dialed it, it was great. My only complaint, and this is during very specific circumstances, is when the rear activates on a certain bump, it stays open for a little bit, and I feel it bob around. I'd notice this while climbing. When it's mostly smooth, the rear is rock solid and super efficient. When I'd ride over a rock or root or something like that to activate the rear, I'd feel the rear end compress and soften the blow from the obstacle. By design of the Brain shock, it's supposed to stay open for another second or two (assuming no new obstacles are hit). During those seconds, it'd bob around. A minor complaint I think. Great bike, can't say what the 06 does with the adjustment.
Now for the Racer X. I recently got my hands on a new 05 Racer X with 06 RP3 shock on it about a month or two ago. It's taking some adjustment (still messing with stem length) but I do enjoy the fully active nature of the rear. I know how it will react at all times, and it doesn't have the suddenly open feeling and bobbing sensation. Overall, I can notice some bob when I look down at the rear while climbing, but if I don't look, I certainly don't feel it sapping away my energy. So far, great ride. Just a few more tweaks and I should be good. I think I will like this bike better because it is almost your perfect balance between cross country racer and trail bike. Not to bash the Epic or anything, I really liked that bike when I had it.
As for fitting, I know there's lots of debate for the proper fit. What I do, is adjust the saddle first. Move it forward or back appropriately in order to get your knee over the pedal axle when in the 3:00/9:00 position in the crank arms. The height should be such that you're near full extension when the arm is fully down @ 6:00. After this is done, then mess with the front end. Try to do only one change at a time and keep the changes small. Changing the stem length by 10mm and changing the height via spacers have major impacts on the ride characteristics.