FWIW, on the acceleration/inertia etc. subject here's my admittedly subjective/non-technical experience. 'Roadified' mtb: Mavic 717xc disc on XT hubs, Panaracer Pasela TG folding, 1.5" (about 410 grams), latex tubes; roadie: Mavic Open Pro on 105 hubs, Panaracer Pasela TG folding 25c, latex tubes. All up weight of the mtb is about 22 -23 lbs, using standard mtb front rings w/ Ultegra 9 sp. road cassette; roadie 50/34 compact with usual cassette, all up about 19/20lbs. Ride position on the mtb mimics 'relaxed on the hoods' on the roadie.

1.Acceleration from stop: BluePig(mtb), hands down. Whether intrinsic to the bike differences, or to do with the fact that BluePig just suits my abilities better, for me the difference is pretty startling. This remains the case through low to middling speeds. However,

2. Once moving along well, this changes to favour the roadie, which I guess stands to reason -- roadies are optimized to handle well/accelerate at speed, xc mtb's to do the same at (relatively speaking) slow to moderate speeds.

3. The other major difference I've observed time/time again is that once up to speed, it is easier to maintain consistency on the roadie, i.e. over rolling roads etc. Here I think the same thing that creates effect #1 creates this: the greater inertia of the larger wheel, due not so much to all-up weight as to the distance of that weight (rim/tire/tube) from the rotating centre, and to the fact that a larger diameter wheel does have slightly less rolling resistance to sharpish-edged surface irregularities (the 'angle of attack' effect).

So, for me/my conditions: BluePig in the city/MUP etc., roadie on longer rides out on open road.