The N900 is a great device, but the E71/E72 is a more well-rounded device and would generally be considered as "better" by the standards most people would rate a smartphone. Coming from the E71 to the N900, I can say that I could easily recommend the E71 to any one of my friends, family, coworkers, or people on the street without hesitation and without knowing their level of technical experience. I can't say the same for the N900, which I could only recommend to maybe one or two of my friends who are more tech-savvy and adventurous.
N900 = superior web browsing, bigger screen with greater resolution, much more customizable, better for media, much funner for those who like to tinker under the hood
E71/E72= great form factor, best keyboard, easier to use, battery life is awesome, bigger selection of natively supported games and useful apps, better email integration, better GPS navigation