The 7800 is a true dual band radio (in that it receives and transmits on both 2m and 70cm), but only has one tuner. The 8800 has two VFOs, so it can tune two bands simultaneously. I have a 7800 and like it a lot, but having two tuners would be nice. But my $0.02 is that the 8800 isn't quite as friendly to operate while moving, so I decided to live without it. If Yaesu made the 8800 with the lighted buttons and big display of the 7800, I'd think about going that way.
"Silent enim leges inter arma." -- Cicero