Granted I levelled my Klink a while ago, but I found the nebula missions were a great way to get through the first 5 levels of each tier. After that, the xp for the turn ins tanks badly though, so you'll have to find another way through the second half of each tier.
Exploration will lvl you faster but PVP is more fun. I would mix it up to avoid thoughts of suicide via extreme boredom.
Agreed. If you're just looking to get to cap, then PvP I'd suggest exploration, but they get boring real quick, so throw in some PvP, maybe while you wait for the 30 minutes to be up to reacquire the explore quest.