I was really enjoying the Quake Wars demo for several days, then TF2 beta came out, and I have not loaded the QW demo since. So for me it might indicate TF2 is farbetter than QW, and I personally do like TF2 more, but I was pleasantly surprised at how much fun QW was.
Also consider TF2 is $30 (or less if you want the other games with it) while QW will be $50 probably. QW has a lot to it, as far as number of maps, character and item upgrades, etc.. but TF2 is surprisingly addictive and has a lot of replay value for not having the amount of depth and content that QW has.
Time and money permitting, I'd like to own both. They are both excellent MP games. But for now, I've bought the Orange box and that's all I have time or money for. I will consider QW in the future, especially after it is cheaper. I still feel I will get quite a bit more playtime out of the QW demo too, if I can ever not play TF2. 
