The RAF boosting will be faster.
If you've got RAF now, I'd recommend boosting to take advantage of it, as it is only for 3 months, and is an enormous boost over not having RAF.

However, this is a game, so go with whichever will be the most fun.
If instances will be the most fun, maybe level your last set via instances.
Or your first if you prefer.

You can also run battlegrounds, for experience, as another option.

I'd personally want 4-5 sets of five toons, from RAF.
Just to provide flexibility, so you can play whatever PvE and PvP styles you choose in the future.