Prefer KF many times over L4D.
L4D was always a bit lackluster for me to be honest. The fact that it has never been wholly stable on my system hasn't helped, but even when I can get a good few uninterupted games in it is generally laggy, linear and uninspiring.
KF is a much easier game to pick up and play, and for me gives me consistent enjoyment. You're still killing hordes of the undead (I don't understand how anyone can really claim that L4D and KF differ in their essential content) but the weapons are more satisfying, it's certainly more challenging, and for me the maps offer much more variety. Only thing that lets it down for me is the lack of competitive multiplayer, which to its credit L4D does very well.
Whichever you were to pick, however, you should accept that they are both fairly shallow, short-burst experiences. Good for a few games during the evenings but little else. Either is worth £15, though certainly no more, but I'd pick KF without question.