Right now I am switching between an american curve and a flat. I find I get more consistent results with the american curve, and on a bottomless portafilter it will collect more consistently in the middle. When using the bottomless portafilter as a metric for pulling a good shot, a curved tamper is kind of cheating. In general, I find it easier to avoid getting the little holes around the outside of the portafilter where the water can bypass the majority of the beans, which result in an underextration. That being said, I think that a shot pulled well on either flat or curved tamper will both be excellent. There is no way I can tell the difference by taste.