Ones flat an ones grooved. Sorry I use a grooved one an flat tape so that the base melts the glue into ridge lines a bit like an adhesive trowel does when you spread the glue out. Matt will know I ain't really sure which one is the best but I look at it like this a flat base just flatens the tape adhesive ridges into a spread out mess, where as the grooved base atleast goes some way to keeping those ridge lines giving you something to push the seam down into giving a more spread out bond, if that makes sense.
