From what i know from when i used to play, the two handed backhand is more used to drive the ball down to the end of the court to push your opponent around. Or to do those smashing shots downt the line when you've just been having an across court rally.
The one handed though is used for drop shots just behind the net and to put backspin on the ball, BUT when i played in the past i could place the ball anywhere on the court within half a metre of where i was aiming. So i think that the one handed backhand is just for placement, and if your opponent is up at the net, the one-handed backhand should be the better of the two to go up over him/her.
Each backhand is obviously better than the other in different situations but i recon that if i had to play a full game only using only the two handed backhand i would have a much bigger chance of winning than with just the one handed, so i would say the two handed backhand is better.
Hope this helped :)