just sitting on bikes is not going to do you any good. I mean, its better than guessing, but its definitely not good enough to make a good decision. So, if possible test ride a few bikes. Possibly different sizes, makes and models. this will give you a good idea.

I would like to share one of my recent experiences. I was test riding a Gary Fisher Tass and a Trek (can not remember the model name), while sitting on the bike at the bike shop the Trek felt more comfortable. and the Tass's long top tube made me stretch too much. But, when I took them out for test rides - the Tass felt much better (in climbing, turning, energy transfer etc.) than the Trek. Also, things like Stem, handlebar, length of the crank arm, angles of the seat and head tube are important consideration. but, over all test riding is by far the best way to decide about a bike. It would be ideal if you could test ride the bike in the kind of condition you would be using it in, but I have not found a bike shop who would let me take a bike for test riding in real trails. Although, you can borrow your friends bike and test it on your preferred trails, if your friends happen to own the brand/model you want.

good luck.