Ive had both the V-10 (2009 model year) and a 8.1 (2008 model year) in Class C's. The V10 was in a much smaller unit, by about 4500lbs. I prefer driving the 8.1/Allison combo in the heavier unit.
I think the newer V-10's with the updated transmissions may be a different story BUT I really didn't like the RPM the V10 liked to operate at and the constant shifting of it to keep it in its powerband. It looks at a rise in the road in shifts down and rpm jumps. No harm in it, just not a great driving experience.
The 8.1 / Allison combo drives just like I think it should, right gear, at the right time and no crazy rpm jumps to maintain speed on inclines. It will drop gears to CLIMB, im just talking about a rise in the road or slight overpass.
Both got about same MPG.
I wouldn't hesitate to get either one if I preferred a floor plan between two coaches, all things being equal, I would choose the 8.1/Allison combo.