I guess I'll tackle this one...
You're looking at 2 different measurements and assuming that they are one in the same.
18" is the size of the frame, typically measured from the center of the bottom bracket to the top of the seat tube (sometimes measured to the center).
26" refers to the wheel size. Most mountain bikes use 26" wheels, although 29" wheeled bikes are becoming more popular.
Given you size (and having no idea about your legs vs. torso measurements) I'd say you should look in the 18"-20" range. If you plan to do "hardcore" trail riding, you'll want to stay on the smaller end of the scale so that your "boys" don't bounce of the top tube too much on impact.