There will be an argument, but I hold that post-RaF boxing is always slower than levelling a single character.

The big win is that you end up with multiple 80s so the "time per character" is far, far lower than doing them individually.

So...

I want to use the Shammy in end game content and get it there as quickly as possible.
If this is your focussed goal, and you don't care that the total time investment to level both characters will be higher, option B is the way to go.

It will give you a level 80 shammy quicker than doing both characters together.