The following pros of the big mark v are the ones that may be material in your decision: extra channel (3 rather than 2), foot switchable reverb, louder.
The following factors you might think of as pros but really they are much of a muchness to: size / weight, 3 additional voicing (notably having the crunch setting on the additional channel), supposed differences in how quick the tubes break up at high volumes (although this could be a good or bad thing), additional output master, more tweak ability.
I imagine they will sound very similar. As in, you'd have to work for Mesa or have them side by side to tell the difference.
The following factors you might think of as pros but really they are much of a muchness to: size / weight, 3 additional voicing (notably having the crunch setting on the additional channel), supposed differences in how quick the tubes break up at high volumes (although this could be a good or bad thing), additional output master, more tweak ability.
I imagine they will sound very similar. As in, you'd have to work for Mesa or have them side by side to tell the difference.