I have a passionate love affair going with the .54 and patched balls in sidehammers. For me it's the "bestess".

But I have no desire for a .54 in-line. If you're going to shoot pistol bullets and sabots, the .50 makes more sense to me, or even the .45 with the right twist and bullet. But I believe success in an in-line is easier to achieve with the .50 simply because of the pretty much standard 1:28" twist and the availablity of a variety of components.