For the most part, I prefer dubs. I find they're better at conveying emotion than subs...although that is, admittedly, probably due to the language barrier. With the exceptions of Norio Wakamoto and Mamoru Miyano, literally all Japanese voice actors sound the same to me, and the only emotions I can really recognise in their speech are the extremes. Since I actually understand what the characters are saying in the dub version without the need for subtitles, I pick up so much more from what is being said, and I appreciate it a lot more.
...that said, the downside of that is the bad voice actors. I'm pretty sure I've covered which voice actors I don't like elsewhere at some stage, so I'll skip the list, but suffice it to say that when they appear - which they often do - my enjoyment of the show drops several notches. Dubs have the potential to be atrocious, as well as amazing, regardless of how good or bad the story is. Code Geass is a show I despise, yet the voice cast is one of the best I've ever heard. I adore Gurren Lagann, but I loathe the dub voice acting. Voices can really make or break a show, and with subbed anime I don't have that problem, because the voices are going to sound the same to me regardless, so I can focus on the plot/action/soundtrack/whatever else.
Plus, 90s dubs are awesome. You have no sense of fun whatsoever if you don't smile at the so-cheesy-its-awesome opening themes, or the blatant disregard for the original script as characters break the fourth wall, make inane comments about this and that, and generally act more like it's an Abridged series than an official adaptation. Without Cardcaptors, Sailor Moon, Pokemon and Digimon, I doubt I'd be as interested in anime now. I owe a lot to dubbed anime, and I'll admit it upsets me a little when "sub-elitists" dismiss them without even bothering to watch them just because they aren't Japanese.
So, where a dub is available and doesn't have an individual I detest in a prominent role, I will watch a show dubbed, otherwise I'll go subbed. If it's just been released and I'm too impatient to wait for dubs, I'll watch it subbed as well. It's a variable thing for me.