I'm sorta not sure at this point which is my favorite of the two, for I find both quite enjoyable. Both have aspects that I like over the other within in them, and thus it makes it kinda hard to decide when it comes to an overall of the L4D series.
When it comes to atmosphere, I tend to prefer the feel of the original L4D campaigns. The dark, claustrophobic environment really appealed to me as I was playing through it. Not saying that the atmosphere of L4D2 was bad or anything, but it was a lot...brighter, you could say. Not just in the sense that their is a mammoth load of sunlight, but that you have a clear goal on where you need to go for rescue. L4D you sorta just bounced around in the dark hoping that one of these options will prove to finally rescue you.
When it comes to game-play, however, I tend to prefer L4D2. It has a much more refined feel of play, and a lot more variety on how one can approach tackling on the campaigns. In L4D all I really ended up using were double pistols and the assault rifle < occasionally, but rarely, the auto-shotgun>. In L4D2 I have a wide range of secondary and primary weapons that I can choose from depending on my mood and how I want to slaughter zombies.
When it comes to the Survivors...it's a toss-up, really. I don't hate either one of them, although I tend to prefer the original Survivor cast just a bit more on the overall scale. However, I like and enjoy both Bill and Nick equally, so I can't really decide. They're both great groups of Survivors to work with, and their interaction in The Passing was quite enjoyable .
And I went on a rant about that, but that's what I get for writing a post after finishing Calculus work. But hey, there's my thoughts on the L4D games for y'all to read < or pass by because it's a large chunk of text, wouldn't blame ya>.
