not sure about the mtb question – I would guess the cross bike would be a better tool. maybe a light rigid race mtb with the correct tyres would have a chance?
Yeah, this does intrigue me… TBH, my rigid mtb is pretty bloody nice and my cross bike is a fair bit lower spec so that causes weight weirdness but even with my normal mtb tyres on it, it’s not that much slower. Would be interesting to try it with skinnier tyres and see what the difference is (in the hands of a knobber anyway)