In theory, smaller wheels will have more rolling resistance both due to greater angular velocity on the bearings, greater tire deformation at the contact area, and less ability to just roll over and ignore bumps. Realistically, those tend to be pretty minor factors, and small wheels also carry benefits like lower rotating mass. Smaller wheels intrinsically lower your gearing, but this can be compensated for with larger chainrings. Ultimately it doesn't really matter that much.