This, if you can believe it, is another old pasty controversy. Sliced potatoes, it’s thought, release more moisture as the pasty bakes, creating a more succulent “gravy” in the interior (especially when half of them are placed on the top of the filling). Also, when laid down over the crust, they’re thought to protect the bottom of the pie from getting soggy as it bakes. The second point makes some sense to me. As to the first, I can’t see where a small cube of potato would release any less moisture than a slice, since I’d think it would have at least as much surface area relative to its weight. Here I’m going to defer to experience, since most of the pasties I ate in the UK (at least of the non-commercial kind) were made with sliced potatoes.