At the risk of getting the finger wag... I'll bite...
First off, it all depends on what you feel like playing and what you have at your disposal. Warzone is red team vs blue team, with reqs and AI enemies thrown in. You start at req lvl 1 and work your way up. I like it as you get to warm up in the game by unlocking stuff instead of getting completely wasted by the other team right off the bat. But the population is really low right now, so it's tough to find a match. XP for regular warzone is roughly the same as firefight... depending on the boost I use, I can get around 9k per match, and they last anywhere from 15-20 minutes or so.
Assault is at a minimum 5 minutes. You either assault and try to take the other team's bases and eventually their core, or your defending your bases/core from the other team. Like regular warzone, you start at lvl 1 reqs and work your way up. Not sure what the XP is like as I haven't played it in a while (not my cup of tea)... it's only around once a month and is a featured playlist, so the xp should be a little better (consult the playlist calendar).
Turbo is chaos at its finest. It's regular warzone... except you start with lvl 9 reqs. It's a blessing... and a curse... I'm horrendous in PVP matches so I hardly waste time pulling reqs. XP is better because like assault, it rotates in once a month (last weekend of the month), and is a featured playlist... so there's that.
Like I said, regular warzone is my personal favorite because it starts out slow and helps me get in the swing of the match (warms me up if you will), and if you put a boost on, the XP isn't bad either. It's also a good place to work on various commendations both personal and for the company. Only thing I don't like about it is with the low population, it's sometimes a struggle to find a match. But again, it's all on personal preference... I'd suggest playing all three and see which one is your favorite. Warzone is around permanently, while assault and turbo rotate in for a weekend and they're gone again for a month... so if you like a change of pace, or something different, maybe those will be up your alley... the choice is yours spartan.