Depends on the size of your turts.
You don't want the currents too strong for your hatchlings, because they can get tossed around pretty easily. Bigger, older turts (I'd say 6"+) would be ok with the current. I've even seen Ralph "surf" the current a couple times.
On the same note, I'm sure your turts would appreciate a place with low or no current to sleep, especially if they're sleeping on the bottom of the tank. Imagine waking up in a different bed then you fell asleep in!
With the two xp3's, I don't think you'd need the powerheads, but I've never used powerheads before myself...