A downcut cutter will give you a clean top edge. I have found that they do not work well alone, however. Because they have a "down" spiral, they do not clear chips at all. In fact, they force the chips to the bottom of the pocket. This can so jam the cutter that it breaks. (Guess how I know). I have found that I get the best results if I do an initial pass about 0.02" (0.5mm) undersize with a conventional (up-cutting) cutter, then make a final pass to finished size with a down-cutting cutter.