Since Nintendo Network can bring downloadable retail games (like the next 3D Super Mario), it could be a killer feature.
As for Xbox Live arcade, well Microsoft has a weakness on it. Since Microsoft doesn't want to publish games that have not appear on their service first, Nintendo could bring some indies games on their service exclusively which would be another reason. That's why Cave Story (Wiiware, Steam) or VVVVVV (3DS eShop, Steam) are not on XBLA. http://www.eurogamer.net/articles/2011-08-24-why-microsoft-wont-publish-psn-firsts
Nintendo just need to have all the XBLA library except 1st party games and have more exclusive by touching this weakness.
Yeah Microsoft's "All XBLA games must have a publisher and if you don't have one then we'll act as publisher but you'll have to play by our obtuse rules for the privilege" policy is really ass backwards in this day and age. All other services let independent developers "self-publish" which is why they don't have the same stupid restrictions.