How you know that Motorola milestone has future support? Has Motorola released full roadmap for next n+1 android updates for device?

Only thing you probably know is that there is going to be 2.2.

If you like to test stuff and are not afraid to flash your phone once in a while then go for N900. There will be Meego also Nitdroid is looking pretty good already. Test, devel, enjoy freedom to do almost whatever you want to do (including breaking OS) then choose N900. If you want mature OS with good phone features then choose Motorola.