I have used my N810 an half year, then the display break. So i had to wait 3 months for my N900. I never had a direkt comparison but in my opinion,the N900 is an faster an more powerful N810 with better multitasking (i like animations). I have no problems with the smaller display size.
The features like fm transceiver and ir-transmitter makes it perfekt.
And more users brings more developers