I don't expect the Nexus 10 to sell as many units as the Nexus 7...primarily because the Nexus 7 is obviously much less expensive so can fall into 'impulse purchase' territory. I do though think the Nexus 10 will become the top selling 10 inch form factor Android Tablet since it has the best specs in its class. I'm hoping that Google makes a concerted effort to use the Nexus 10 as a technological 'carrot' to induce developers to really optimize their apps for the tablet form factor.
This is really the last piece of the puzzle that must come in place for Android tablets to really 'win' over the iPad long term. It is the primary reason why even though Android phones now have 75% of the smartphone market, that Android tablets have not kept up with that success so far (although a recent study did show 41% of tablets are Android now so they are gaining steam). Once the 'tablet app' gulf is filled I expect Android tablets to have larger market share than the iPad within a year or two.
The tablet world is at an inflection point. I think 5 years from now Android will have 80% of the tablet market and we'll look back and see the introduction of both the Nexus 7 and Nexus 10 as the 'turning point' in Google's efforts to fight the tablet war. Now that they have the hardware...they will turn their efforts to continue to build out the ecosystem...meaning tons more content available in the Google Play Store (music, movies, books, apps, etc) and optimizing apps for the tablet form factor.