I'm a few years late, but..
I haven't played Borderlands, ever, but I can give my opinion on L4D2.
I have played both L4D and L4D2 and I think that the first is much better. It's has an eerie feeling and I prefer the characters. The only addition that I could really see between the two games were a few game modes and the addition of melee weapons. So, um, L4D > L4D2.