Originally Posted by Limba
Ok so say you have a 19 pound hardtail mtn.bike with a rigid fork and Conti 1.5 tires.Let's say you use a double chainring in the front like a 48/36.When you shoulder the bike the bottom of the saddle sits on your shoulder instead of a cross bike's top tube.How much of a difference now?
Definitly getting better, but I find that my 30mm cross tires always roll a lot faster than my mountain tires (admittidly only 1.9 semi-slicks) - I think the course layout probably decides how much of an advantage it is. A course with a lot of pavement is going to favor the better rolling 700c wheels more, wheras some courses might actually be faster with the mountain bike (causing some consternation among cross purists). Carrying the bike (as I often have to do) the lighter bike is really nice.
Depending on your wheels too, you might end up carrying less. My cross bike has fewer places to collect mud than my FS mountain bike, and the wheels (cosmic elites) V section sheds better, so I probably carry less mud overall.
However, all of this is assuming identical riders, etc. Truthfully, the engine makes things go, and as many in this forum have pointed out, the weight/style/ride/material of the bike really is secondary to the engine. Ride what you can afford, and are comfortable on.