Even if you discounted MeeGo as a no-go (no pun intended) as I did before playing with the N9, Android would make more sense - they already had Linux kernels running well on their hardware and had them for ages. Building Android on top of that would be trivial. And quick.

Nokia could also be the only Android phone maker that would be completely imune to lawsuits by any other phone maker. And from Microsoft ("It's a shame you are suing us, you know. It would be horrible if we decided to sue all your WinMo 6 and 7 OEMs").