Either or. I don't see any particular reason to do either over the other that isn't balanced out by other factors.

If you do them now and continue leveling, you'll take longer in play time than you otherwise would but will hit the expansion content sooner.

If you don't do them now and are playing FFXIV anyway, then you can level other classes or something similar. You'll end up with less play time to reach the expansion content but have to wait longer, too.