Best Ad Server-OpenX, AdSpeed, etc?

I am launching a content site in the next month or so and of course early on traffic will be low.
I’m looking at a solution for an ad server. I had mostly ruled out Google Ad Server but am looking at both OpenX and Adspeed. With Open X being free for up to 100 million impressions the price is right.
Is one of the above ad servers better than the other?

I would recommand you also Google AdWords and Adsense if you want a fast growth.