Although you may know the basic plot and spoilers form the first, you are missing out on getting to know some of the characters from the first. Properly at least.

I found there to be a lot of atmosphere in the second one due to the knowledge I had of the characters and levels from the first, that simply wouldn't have amazed me as much, or left me to see the developments that had gone on.

On the other hand, no matter which way you do it, you'll still be playing a truly awesome game that is thoroughly enjoyable. I just think it will be that little bit better if you play the first one.