Just like King Diamond said.
The Mk II has 3 channels (Rhythm Clean/Crunch, and Lead), a seperate EQ for the rhythm and lead channels, and a better clean sound. Some people say that the Mk 1 has a warmer tone, but I've never heard that myself.
Yes, the Mk II has 6 pre-amp tubes instead of 5. I think the MK II will get down at lower volumes a little better than a Mk1. At high volumes I don't think there is a noticable difference in the tone. For all these reasons I prefer the MK II.