The AdWords traffic is just too broad and "general public" and it's incredibly difficult because to get good CTR (click-through-rate), you actually have to find keywords that produce bad or irrelevant results for the searcher while being on topic, as otherwise the searcher will simply click the organic results and never even see your sponsored result (when was the last time you went to Google, did a search, got good organic results, and decided to skip that and just go to the sponsored link?).
AdCenter is a little better IMO, but niche websites are absolute killer since you're no longer screwing around, trying to filter out irrelevant traffic, trying to get the right keywords and bid prices, and making sure your landing pages have the right quality score. The moment you get a low CTR is the moment Google AdWords start increasing prices, stops showing your ads.
Go right to the source to test your ideas. It will take more money, but you'll get a confidence level of 95-99% for your validation, rather than something much much less that you'll get using AdWords. It will be quicker too; AdWords keywords can be incredibly difficult and time-consuming to get right.