If you don’t do Wetwork first, it’s going to take quite a while to get its perk. Keep in mind that finishing a specialization takes about as long as it did to go from 40 to 50.
You can only use Pioneer’s Fast Track once you’ve finished that specialization, and it takes up your tactical package, so you probably won’t use it every game. Also, Fast Track really doesn’t give you so much XP that you fly through a specialization.
If you do Pioneer, then Wetwork with Fast Track on every game, it’s still going to take a while.
Unless you really want to level up faster, I’d suggest that you knock Wetwork out first, then do Pioneer.
You get the specialization’s unique ability once you’ve reach lvl 10 in that specialization (max level per spec). Then it unlocks the Tactical Package OR Perk related to that specialization. If you use the Fast Track, you can use it towards levels in other specs.