Running both (centre and front) will make for a smoother car to drive. Effectively, the front belt won't spin off power (reducing drag), and you reduce the chance of any difference in the freeness of the bearings of the front one-way unit having an effect on the car.
If you run the direct centre unit, you'll have better throttle response, but you are relying on the front one-way unit to do all the work on turn-in, which can cause problems if the bearings are not close in freeness.
Personally, if I'm running a front one-way, I'll run the centre as well. I just prefer it that way.
For a Diff or Spool, I'd stick with a direct centre unit, simply as otherwise you loose the main advantage for running one of thoose options... braking ability.
HiH
Ed