It's not exactly an either/or question. Some English subtitling jobs are pathetic; others are works of art. Likewise, while some original Japanese dubs are very well done, despite what a certain type of hardcore "Anime elitist" would like you to believe, not all Japanese dubs are works of peerless mastery of an art form the Gaijin could never hope to equal.
In particular, the American release of "Princess Mononoke" was a very careful piece of craft. They got Neil Gaiman to adapt the English version of the script, and a top-flight cast to do the voices, including Gillian Anderson, Billy Crudup, Minnie Driver, and Claire Danes. Yet a certain otaku hardcore still screeched to the high heavens that their precious Mononoke Hime was being defiled by this version.
So... Here's the thing. A really good anime with an English dub is going to be seen by a hell of a lot more people than the same anime is subtitled. This is why a company like Disney deigns to do a major theatrical release at all. And I'm grateful that when they do such a thing they go through immense pains to do it well. It pains, disgusts, and embarrasses me that certain anime fans would rather keep their hobby a shadowy niche than allow it to be available to people who would appreciate it.
It's one thing if there's a danger that the original Japanese dub would cease to exist, but that's hardly ever the case. And, again, not all dubs are equal; if a anime is going to have a relatively narrow DVD-only release, I'd rather read subtitles than hear the key grip's brother try to act. But for the big releases? Dub well, and you absolutely have my blessing.