Android is open source, however, Google still controls what you can do with it. Maemo however, is completely open to whatever you want to do with it. You can even open a terminal and use it as if you were at your home PC. You can download and run any app that will run on Linux. The only real drawback right now is that the phone application Nokia includes to make calls on a GSM network is limited. But to me, that's ok. Android is pretty awesome, and it offers some really cool features. But I personally am rooting for Maemo. I'll be getting a Nokia n900 when my sprint plan expires and I can move to T-Mobile. Just my 2 cents.
P.S. The two aren't really competing, as Maemo is meant more for an internet tablet, not just a phone. The n900 is marketed as a mini computer with phone capabilities.