Should I buy L4D or L4D2

buy l4d2 you can play l4d maps in it plus it was free so more players if i where you i would buy 4 pack trough

L4D2. There isn't a single reason to buy L4D over L4D2. As stated above, L4D2 has all of L4D's maps and characters, and is better, and has more players.

Indeed, Left 4 Dead 2 ported all of the Left 4 Dead 1 campaigns. There's no real reason to get Left 4 Dead 1, when 2 includes so much more content.

l4d2. source 2 is coming for it
rumor

I for one always like playing the first one before the second to me it's like starting to read a book on chapter 2

L4D2 includes all the L4D1 content so it's pretty clear.

I like the first one more. It has slightly different mechanics, music, etc. It feels very different for me, has better atmosphere (imho). But yes, there aren't many people nowadays in the first l4d.
The second part is more like an attraction. Technically it includes all the campaigns from the first part but I didn't play them there.

I would recommend you to buy both parts if you really enjoy the game and have people with whom you can play the first l4d together. Else go with l4d2 (even that I don't like it that much).