Which Booster should I use in SFK?

After reading all of Ualaa's great posts, I get the A->b->c->d->e Makes great sense. I'm fortunate to have a 80 DK (Epic Tank Gear), and a 78 (Feral Tank Spec - full Heirlooms, etc) Druid on Account A, I also have a level 50 SPriest. All which could boost acconts B...E.

Assumption:
The 50 SPriest will not nerf the XP to the other 4 toons nearly as bad as the Druid.

Question:
Is my assumption correct? Given my situation, would you boost accounts B...E through SFK with the 50 priest or the 78 druid? Is there a formula somewhere for how much the 'nerf' penalty is per level?

The nice thing about the druid. I can basically clear SFK in 4 pulls.
1 - Opening area.
2 - Courtyard.
3 - Up the stairs, all the way around and back down to baron and out the kitchen
4 - The rest, up to Fenrus.

The bad thing with the druid, I can easily clear 5 times in under and hour....pulling mostly in cat form.

Your alts will get the same xp if you boost with a 50 or an 80 as long as they are raf linked. Use your druid or DK, it will be easier.

Thanks for the reply!