Both will factor in. Slick vs knobby being the biggest difference, then the width of the contact patch comes into play. Air pressure will be a determining factor in this as well.
I'm on a set of Schwalbe (sp?) CX Pro in 26 inch ... cross style tires @ 1.4 inches, and Micheline XCR's are one of my favorite MTB tires ever, just awesome fast.
http://two-wheels.michelin.com/2w/fr...183349&lang=EN