I dunno what clam shell is, but X-car wins for me because it is so versatile. It just really needs to be put on some intamin megacoaster or any b+m, as then airtime coasters can become multi inversion aswell. Also, they could be used on dive machines, as the Ostr's on oblivion slightly comfort you stopping the fear of the drop, and the are not very needed because there are no inversions anyway.
So yeah, I'd go for x-car restraints, but mounted on an intamin or B+M train.
...oh, are clamshells b+m lapbars? They are probably one of my favourites because they are so damn big and comfy. Didn't feel so much airtime on S star as I did on EGF though... but that's probably the track.