They both have their Pros and Cons.

a. G has better ad targeting (IMO) and good ad inventory. Y seems to have less ad inventory but at times seems to have a better CTR (but not always).

b. Another issue which I find important is if you have more than 1 Ad Unit per page (commonly done) G puts different ads in each ad unit block, but Y normally duplicates the ads with the same ones appearing in all the ad units on the page.

c. Y has the advantage of a much more personal level of support. Not necessarily better support but more personal.

d. G can do payment by electronic funds but Y only mails out checks.

e. G can handle foreign traffic and even foreigh language web-pages but I do not think Y does that or even allows it.

Even though I have and use both accounts I still keep most of my traffic going to Adsense, with somewhat of a bias there, mostly due to reason B above. Actually I like both of them as they are both excellent programs.