I don't prefer anything. If it's the only way I can watch the show, then I'll watch it with subs. And when it's dubbed, I'll watch it dub. Really, only 4Kids shows have bad dubs, others like Naruto, Funi One Piece, FMA, Cowboy Bebop, Champloo, and many others are actually done pretty well, it's just that some people don't like it for stupid reasons, and then everyone jumps on the bandwagon to be cool.
So basically, I'll watch Naruto Shippuden subbed since I have to, but as soon as it's dubbed, I'll never watch that sub again, mainly because Shippuden sucks ***.