Alot of times now days the english VAs are picked by the creators in japan, so often times if your not prejudiced towards the purest form of their voices, at lest for the main characters, they match the japan voices pretty well.
Some anime are just hard to read the subs without pausing, code gesss comes to mind there is a ton of dialogue and sometimes it overlaps making it hard to read without pausing the thing every 30 seconds just to catch it all, so i prefer code geass dubbed, even with the horrific nunally actor, since i am not big on pausing things just to read it all.
Subs do tend to me more accurate tho since they often change phrases and wordings around to lip synch it better, old animes were horrible tho dbz and the like comes to mind, you could watch it sub and dubbed and come away with 2 entirely different understandings of the plot and the specific details. Newer animes tend to try to be more faithful however and do a better job of trying to keep things in line.
Often time for more popular animes it seems they kid the dialogue up, naruto springs to mind where the fan subs have a good bit of the f word and general cursing and such, that is entirely cleaned up for US consumption, now maybe the fan subs take some liberties i dunno since i have no clue what the f bomb sounds like in japanese. But considering that animes are usually targeted at more adult and young teen audiences in japan where over here they are aimed more at children, it makes sense that the japan stuff would be more adult period.
I think now days the dubs are way way more accurate overall and there some really good voice actors working for the english ones tho so i think it just comes down to personal tastes and how well you can stand the less talented VAs.