RAF exp or boost?

[Stabface]

what is the fastest: dualboxing with same lvl char or boosting.

If you are appropriately classed/skilled/geared at boosting, then it is faster than questing. Boosting with a Resto Druid or Holy Priest is overall probably going to be slower than questing.
Boosting with a Mage/Paladin is somewhere around 1.5x-2x or more faster, in my experience. The /played numbers people have posted up hold this to be fairly universally true.
Therefore, until Blizzard fixes the RAF bonus so that it no longer works while boosting, then boosting+RAF will be faster than questing+RAF. Once it is fixed then questing+RAF will be faster.

Also there's a ton of good threads on boosting already that give you some ideas of what is possible.