This is really a question for Adobe rather than the community, but my understanding is that the AEM as a Cloud Service is functionally very similar to the latest 6.5 release of AEM. So it's not as if there's a 6.6 version that is only available in the Cloud Service paradigm and all of the AMS or on premises customers won't get the latest and greatest functionality until they migrate.
AEM as a Cloud Service does have some beneficial differences from the current AEM paradigm--but mostly on scalability and automated deployment of patching. In other words, the differences and benefits are mostly on the back-end technical side rather than use facing.
There may be an eventual world where AEM on premises customers are forced to migrate in order to get new patches and functionality, but I'd suspect that's at least 2 years out if it ever happens.
Hope this helps,
Beau
