If you're on a pvp server, pvp is a threat all the way up.
If you're not, then spec whichever is the fastest way to level.

For PvE, that's generally Affliction until you get Felguards.
And then it depends on whether you're taking fewer harder/higher things or massive lower things.
Either approach is a very fast leveling pace.

If you're going the BG leveling route, then a PvP gearing/spec is essentially forced on you.
The same as playing on a PvP server.

A drain team, something like SL/SL from a couple of arena seasons ago might be good in that case.
Soul Link for extra survival, and Siphon Life (as part of your Corruption dot).