DearS, the ainme, is nothing compared to the DearS manga- given that Khi and Rubi got more screentime and a more intimate relationship with each other, but all of the fun, deep, emotional parts of the series were lost in the anime.
I say; you might want to watch the anime first, because if you have read the manga first, you'll be heavily disappointed in the Anime.
I had read the manga, eventually found out there was an anime, watched it- and was very and very disappointed. Many of my favorite parts were gone, and i could definitely see DearS being just as great an anime as Rozen Maiden or Chobits, if it had been given more of the actual story from the books.
A Huge bit of story missed, so by all means- watch the anime, but also read the manga, since all of the good things are inside of the manga.