I will be getting Nexus next to my N900 while waiting for the MeeGo device.
Best example i did read was in a finnish forum where a guy said that when he is on a move he usually takes Nexus, when there's no hurry he uses N900 to browse the net, listen music(multitask) etc.

So the tablet bloodline shows.

My educated guess would be that N900 got better camera, multitasking is done better and browser is better, especially if you have overclocked N900.
Nexus One is sleeker, Android could be better option depending on what you want, better portrait support, more apps and like N900 there's viral community for hacks and what not(something i find important).