Through Adword you can advertise in any site with Adsense. Practically all 90% of all of the sites with advertisements. With google screening Adsense users i think you might get better conv. ration though Adwords. I have used Adwords but i never used Adcenter so i can not do a comparison. But I can tell you that you better have good funds if you are starting an ad campaign on GAdword
I'd say adwords was better for me. Google is a little more expensive with the keywords I was trying to compete with. It was worth it until I got my site established.
Yes, Bing is getting a piece of the pie there, the UK as well. I have talked one of my clients into trying AdCenter in addition to AdWords, will compare the results soon.
where can i see the numbers of traffic managed by adcenter? i would like to know if they get a lot latin america traffic