Overall I'm with PW on this. I want the safe / more consistent flip to get the best chance at a lead runner getting out. As we all know getting that lead out is super huge and getting the second out is icing on the cake. But if you get good at it then you'll turn more double plays.
We do a drill that is great for working on double play flips, 1B flips to P and (it's a stretch) 3B flipping to C on bunt). We take all our IF and put them in a four corner box equal distance away. One guy starts with the ball and he starts down on the ground with ball in and throwing hand on top of ball (basic fielding position). They will work the flip to the next corner. Also, like PW says, they have to follow the flip to help with accuracy and getting it there with something on it. We don't let them take their arm above their shoulder because that is what puts humps on the flip. The corner receiving the flip will say "flip" as the ball is in the glove on the ground. We want the receiver to call out flip to get a flip OR say nothing and it will be a throw. This can be a pretty fast paced drill where you can get a ton of flips in a short amount of time.
Now if a kid comes to me who can do the backhand flip and is good at it by proving it in the drill as well as the other things we do in practice I'm not going to change anything. But overall I want a catch, pivot of the feet and underhand flip to the SS.