It's personal preference, really. Most people will automatically say MW2, but if you like L4D more than COD:MW, then you should probably get L4D2. Both games should be great.
From a CAG standpoint, I don't know that MW2 will drop significantly in price after the initial $20GC offers from Amazon & Walmart.com. L4D2, however, will be $40 on Black Friday. I'm getting both, and I'll only essentially be spending $80, which is quite reasonable for new titles.