Ride the biggest one your bike will fit. If you’re a racer you aren’t reading this, and if you’re not, this is great advice. The greater volume of air more than makes up for the slight additional weight that comes from a fatter balloon.
Forget thin sidewalls. Don’t worry about supple sidewalls and any claimed (even if slightly real) reduction in rolling resistance they may provide. Listen: Flexy sidewalls fatigue more and get weak, and a blown sidewall is serious. Thick sidewalls last longer, are safer, and have the tiny remote once-in-a-lifetime advantage of being way easier to ride on when your tire is dead flat and you’ve got no tube. It’ll happen sometime.
Check sidewall wear often. In modern tires, the tread will far outlast the sidewalls, and this is especially true of tires with thin sidewalls.
A little tread seems to help on wet roads.
For recreational (not racing) riding on trails, volume trumps tread design. Go with the knobby, sure, unless the knobby is smaller than the fat barely-knobby. Ride within the tire’s limits, and learn what they are.
Choosing a selection results in a full page refresh.