The N900 is still a sweet device with what's still in some ways a more modern tablet and touchscreen aware OS (Maemo). The N8 has a lot more in common with Symbian devices like the N97 and is a bit more awkward in terms of UI than Maemo, iOS and Android. But you would get a newer version of Flash with the N8, something that likely won't get updated on the N900. Personally, I'd stick with the N900 if you can live without the latest and greatest Adobe Flash support.
-------------------- Lisa Gade Editor in Chief, MobileTechReview