While the new areas will be locked behind the last expansion, you'll be able to pick up the new jobs, once your regular character hits level 50 and finished the old ARR dungeon/msq aka praetorium and castrum.

That gives you two options since SB includes the HW expansion as well,

a) go ahead right now and sub, play the whole HW expansion, then all you've to do is level the new jobs once they are released without having to unlock everything (farming gear on the way etc) or

b) wait. If SB is anything like HW was, you'll need lots of fates and side quests and probably have to spam the new unlocked dungeons, to even get close but still not fully to 70.

With that in mind, if you would wait and take your level 50 throughout the already buffed HW area, you'll easily 64 by the time you reach the SB areas and could probably without frustration get to 70, compared to us people that can't wait and "love" the grind.

I personally if I would have taken a break, would chose option B. But since I didn't take a break and already being bored for the last 9 months, I'll take option A and kick myself in the ass for having even considered that route. Chose your path wisely