You want people to have positive feelings about your company (you are putting a lot of work into it in order to help people, right?) so drop the negative advertising. Figure out ways to differentiate and complement others so that people who use and like other services don't feel personally threatened by your advertising.
Even if it is really obvious you are competing with someone, don't say it that out loud, since you are likely to fork into a different business model from them as you grow and find your own niche. Playing nicely with others will get you more positive buzz from the community, and you'll be perceived as a better partner by others if you play nicely.:
"Adsense for X, Wozad for Y".
Also, if you want to fight Google head on you're likely to lose. Better to find a niche, start making money and use that to improve your service.