Regardless, to the average end user this distinction doesn't matter. I think users will gravitate to what works well and what their friends use.
I really hope this is Google Hangouts instead of Facebook Messages (ugggh).
iMessages has problems of its own (TERRIBLE sync between devices), but it has the huge advantage of working for everyone with an iDevice.
What I really want:
1) a service that works seamlessly between any device (like iMessages is supposed to for iOS and OS X)
2) this service should be platform-agnostic, working on any computer or mobile device
3) it should be federated like Jabber so we don't have to depend on one company
4) comparable features with the existing services: read receipts, group chat, push notifications
I hope Google Hangouts is exactly this, and that they let me enable it for my Google Apps account.