I have a couple of other things to say about this comparison, not about quibbling over who likes what, but as far as the nature of the games. Team Fortress 2 is typically played with team damage off, and L4D with team damage on. Team damage is a huge factor concerning whether a person can casually enjoy a game with a random group of people.
Hard core gamers normally scoff at having damage off, but it seems very acceptable in TF2 because of the other complexities that keep the game strategically interesting, and the relatively casual nature of the average TF2 match.
In L4D, team damage is likewise fitting, and it varies a lot depending upon what difficulty level you play in campaign modes, as well as in versus, where the damage isn't relatively high, but is always noticed and has a cumulative effect. Also worth noting is that the small number of players combined with the inherent occurrences of observer mode time (particularly in Versus mode while playing infected) cause players to tend to make acute judgements about how each other should play.
On the other hand, in TF2 there is (usually) relatively little scrutiny about what you choose to do or try, apart from general remarks such as "we need another medic" or maybe "we should have an engineer" etc. In short, what I'm saying is, both games have settings that are fitting to them, but if you are a Type A personality who wants to have fun without added stress, TF2 is a safer choice. If you fancy yourself more of a hardcore player, maybe you can enjoy either, though TF2 feels less serious.