As for mixed fleet flying, Thomas Cook Airlines in the UK still MFF with the A320 family and A330 family, varying in MTOW of 77,000kg to 230,000 kg.
When MFF its different but still the same to fly, basically one you sit in for about 5 hours and the other around eight hours, you still get a sore butt!
The 777 and A330 are both good at what they do but the A330 on looks is the winner for me.
With regard to the A330 v the B777 as a MRT Tanker - why not buy both, then it would keep everyone happy provide different capabilitiy and if one turns out to be a dog then ditch it at a later date, history shows that the US Military used to do this all the time, especially the Navy, if they didnt like an aircraft they would give it to the Marines to make it work!
edited: because I cant touch type
