I prefer subbed anime definitely. Japanese voice actors are usually much better, I will concede that in recent years there have been plenty of series that I thought were dubbed well enough to watch in that dub (Gundam SEED, Bleach, Code Geass, a few others) and if that keeps up then great, both the American's and Japanese win.
I still will prefer the subs just so that I can pick up on the nuances that are lost in English, even if all of the American voice actors become competent. Being called -Sama vs -Chan is a pretty big difference that takes some creative writing to convey properly in English. It can be done and done well, but there are always compromises.