Boosting will probably be the fastest, especially if you have even a half geared 80th booster.

If you went:
A >> B >> C

You boost BC with A, from 1st to 60th.
Boost a second set, also to 60th.

That gets you, a full 60th on A, and a full 60th on B.
The granted 60th on B, gets you 30 more levels on A.

Boosting four toons on BC with A, gets you:
Two toons on A from B (and two toons on B from C).
B can then give another toon to A (cascade granted levels).

So A gets three toons at 60th, from boosting four toons on BC.
And each toon takes 1/3rd the normal leveling time.
So in the time it takes to level 1.33 toons (without RAF) you get three toons.




Conversely, if you were to level three toons, on ABC, but do it twice.
All are the same level, so no boosting.

You get two full toon on A, from the leveling.
And another full toon on A from B.




I'd tend to think you could boost four toons, with an 80th, in a lot less time then leveling two sets from 1st to 60th via questing without a booster.

Boosting the four toons gets you three toons on A.
Leveling two toons gets you three and a half toons.