The difference is in philosophy, the N900 is for us geeks a small computer with phone hardware added on.
Most phones start as phones with all of the limitations assumed and eventually built in, and it can do computery functions too.
I love having a computer which has been hacked to make phone calls in my pocket. I wish there were more manufacturers out there making giant tablet-phones who would get that a % of people actually want computers that make calls not the other way around.
The truth is the polish and variety of many apps surpasses that available on Ubuntu.
I kept using my Sharp Zaurus until 2010 when I got my N900, the Z also survived because even when old it was a real Debian based computer with hardware keyboard in my pocket.