It's hard to choose. If there is little difference, like Bleach, then in English. But when there is a large difference, like Naruto, I prefer Japanese.
It depends on the voice actors if the Japanese Voice actors are annoying, but the english voice actors are not as annoying. I will watch the english anime, or the opposite.
I prefer English since I can sit back and relax since some subs are so fast that it can br hard to keep up.
I much rather have english dub then japanese as I hate trying to look at something and read something at the same time. If I were suppose to read anime then I would just read manga.