The n900 does not have a capacative screen and is more aimed at the geeky end of the market (tho Nokia may be trying to re align this after the n97 debarcle)
I considered the n900, x6, desire & legend before finally plumping for the desire. It has ticked all the boxes I wanted GPS for bike rides a decent screen and performance for watching videos and the like and decent internet.
I think if you are goin gto do really tekky things possibly go for the n900 but if not the Desire.
I think the x6 is still a symbian operating system which is miles off android 2.1
All my opinion though.
Have a look at the reviews on techradar:
http://www.techradar.com/news/phone-and-communications/mobile-phones/