The site I'm working on redesigning (from php to RoR) currently has
phpads, and has paid advertisers that rely on stats to see how
effective their ads are.
Why not keep using phpads? It works well, is a known quantity and can be easily integrated into Rails. We wrote a little helper method "render_banner()" that you can pass a size, zone, refresh, etc to that works very nicely. One of the nice things about it is that we wrap the whole banner into an iframe so it protects our site from those HTML banners that blow the page to pieces.
If you're interested let me know and I can dig it up...