i would think that having a faster swing speed when you're trying to fight would be better for combat but that's just me
Put yourself in a situation where you're getting ambushed, you have greater than or equal to 75% uber
Ubersaw You can hit a enemy and uber your way out (sure there's a risk, but you're gonna die anyway right?)
Bonesaw Unless you somehow get 100% crits you're gonna die, or at least become weakened enough to die to the next enemy you encounter trying to get to a health pack or spawn.
We use the ubersaw cause it's the best weapon that medic has for a melee, and that 25% uber can make a big difference.
(also taunt kill uber)