I've little to no experience on riding mountain bikes but will think that like road bikes, once you get in the range of suggested sizes by the manufacturer, you are okay with either. The smaller bike might feel a little sportier and the larger bike more like a four door sedan going down the road.

Depending on your fitness level and previous riding experiences your position on a bike might change quite a bit. As well your preferences might change in how you want to fit. So just pick a bike, preferably by riding each as long as you can convince them to let you. Then make certain you aren't spending your whole wad on this one bike so if after a thousand miles you find you chose poorly that you have some cash to do something else. Hopefully it's only a stem, saddle, or handlebar change which aren't too much money.

Are you really going to ride this as a mountain bike? Off paved roads? If not, then you might be misunderstanding why they put the word mountain with bike.