depends on your 4x4 style, I would say use the same on both cubes, that'll help getting used to it.
In my opinion r slice is better, if you have got a really good look-ahead and never have to glance at the piece in the buffer position, cause you cannot see that when using r-slice and have to make quite strange moves with your hands to see it.
I, myself, use the d-slice as Arnaud does and I'm quite happy with that, cause I use it for the 4x4, never practiced 5x5 that much, but it feels exactly the same. It is a litte bit easier with a less perfect look ahead.
Well, okay, Erik uses the r-slice, maybe I will switch to that, too, because I'm a bit unhappy with shooting from D while using d-slice. r-slice-technique is more symmetric, but as long as my look-ahead isn't good enough there is no need to switch.