In this case, bigger IS better. Per square inch, the various models of WACOM tablets have the same resolution within those models so if you want higher resolution, you'll need a bigger tablet. They have three or four models and several sizes per model. Each model has it's options that make using it different from the other models. You can assign different commands to the buttons, so if you use several commands regularly, you may want a tablet with more buttons.
I made the transition to pressure sensitive about a year ago and have noticed a difference in productivity. Until that time, I had been using a drafting tablet and puck.
My best advice is to try as many as you can. Even if you can buy it cheaper online, this is a case where I would advise you to find a local dealer willing to let you try one or two before you buy one. Stick with that dealer for the purchase, the extra ten or twenty bucks will be worth it in the long run and you'll have a "go-to" guy just in case.
