I prefer dubs, but I don't tend to watch them because I rarely watch anime old enough to have them. Usually dubs don't come around until a year or two after the Japanese release and I tend to check out anime because of hype surrounding it as it airs.
But I prefer dubs because I don't have to read subtitles at all for them and I don't have to pay much attention to them. I like to watch anime while I'm doing something else (grinding in an RPG, browsing the web, etc.) so having it in my native language is easiest. My Japanese is pretty good and I can usually understand something fairly well without subtitles but I have to pay close attention to everything someone says and I do sometimes have to check the subs to see exactly what's going on. I tend to prefer anime with complicated plots so having it in the background in the original language doesn't work out for me very well.
Eventually I'll get to the point where it won't matter what language it's in because I'll understand it just fine either way but until then I'll pick it up in English whenever I can.