I can't give you information on the engine questions. But here is some information that might help.

The automatic transmission in the M2 will be a Allison 6 speed. In the Columbia you can get a Meritor Freedom fully automatic 12 speed.

The Columbia has a bigger cab. In trying to have a shorter overall height this could be a problem. The BBC (bumper to back of cab) distance on a M2 is 8 1/2+ feet. On the Columbia it is 10+ feet. So with the same motorhome box you will have a total length of about 1 1/2 feet longer using the Columbia chassis.

But the Columbia cab is also taller. If you do not order an extra height body on a Columbia the overcab bunk is not useable except for a small kid. I do not remember the exact height that they add to the body on a Columbia to make the cabover bunk useable but I think it is 12 inches taller. If you can compare a M2 and a Columbia cabs you will discover the difference. Even with the extra height I seem to remember that the M2 still has more room in the cabover bunk.

When I was in Elkhart last May I was told that without the extra height the only use for the overcab area on a Columbia is for some storage cabinets.

So it is a trade off between the extra power with the Columbia chassis vs the size problems. And the Columbia is more expensive.