Originally Posted by zergling_seb
-Follows the Actual Story.
-Gorey (Much gorier than the original)
-Visually superior to the Original series.
-Each episode is about an hour instead of half an hour.
Both the OVA, and the series, follow the Manga's plot. The OVA is just more of a continuance to the Manga.
OVA's are the Japanese version of "Straight to DVD release". So pretty much, they aren't censored. That's why you get more gore.
As far as visually superior, yes. They only had to animate 3 hours opposed to 9.5 hours.
Note: The series is 13 episodes.
Personally, I suggest watching both.