Well, if you guys read a manga and watched the anime version of that manga, you'll see that sometimes there are huge differences.
For example, if you watch I''s anime before reading the manga, that's a huge mistake. The anime is bad and very very resumed. It's just a spoil to the manga. But there cases where the opposite is also true. The Onegai Twins and Onegai Teacher mangas are spoils to the animes.
So, about Gunslinger Girl, should we start reading the manga or watch the anime first? I already watched and loved the GG Season 1 anime. But I heard terrible things about GG Teatrino (at least I can confirm that the new animation sucks!). So I don't know if I should read the manga first, before spoiling the series with Teatrino. The problem is, the manga is ongoing and is taking forever....
Well I also had that problem but i decided to watch the anime before reading the manga and if the anime sucked i would go to the manga. If the manga sucked i would drop the series or i would try the anime if it still sucks drop it.