I think you cant run "Mer" on N900 but Nemo which is based on Mer (formerly MeeGo). Besides you can also run Android on N900.

Which is the best? I'd clearly go for MAEMO + CSSU.
Why? Because it is fast & stable and the UI is very good. Im still dreaming of a 7-10" tablet from Nokia with Maemo 5 because i think Nokia never did take full adavantage of the Maemo 5 UI in terms of of the hardware it runs on. In my opinion its the best UI for a tablet.

Nemo is still in an early stage of developing, but despite it is constantly getting better imho it is still very far away from becoming a usable on a daily basis. I doubt we will see it on any commercial hardware / as a commercial product in the near future.

Same goes for Android / Nitdroid. Some things work some things not. You can use it but its not suitable for your everyday needs.