There's a 3rd option.

A) Upgrade the accounts, level the toons, keep the accounts.
B) Level the toons without upgrading, transfer the toons.
C) Level toons on your upgraded keeper accounts, while grouped with vanilla throw away accounts. Never upgrade the throw away accounts, and lose the leveled toons on those throw away accounts.

Option C is decent, if for example you already have... A, B, C, D and E.
And those accounts are fully upgraded with Classic + BC + Wrath + say Cataclysm.
You could buy 4 throw away accounts, purely for RAF.

Link 2 of them to B and C.
So we have B - F and C - G.
Use a toon on A, D or E to boost the B-F and C-G toons.
Do as many teams as you like.

Then link 2 more to D and E.
So we have D - H and E - I.
Use a toon on A, B or C to boost the D-H and E-I toons.
Do the same number of teams.

Unfortunately, you have to level twice for a new four-some.
The BC-FG and then the DE-HI.
But they level 3x faster, so you're only investing 1/3rd + 1/3rd the normal 1-60 time.
So its still a lot faster then 1-60 on BCDE without RAF.
And the FGHI accounts do not need to be upgraded to BC, Wrath or Cataclysm.
Since they're throw away accounts anyway, they can partner with your keeper Draenei/Blood Elves as any classic race.