I'm pretty sure Microsoft will run it exactly like Sony. The reason you lose your games when your PSN+ subscription expires is because the free games rotate. So, you lose your access to one that was offered 18 months ago, it's deleted, and then it's not offered again when you sign back on because there are 12 (or whatever) new games. It may enter the rotation again and you can snag it, it may not.
As soon as Microsoft announced that they were offering free games, the criticisms were that there were only two of them...and they were really old. Apparently PSN+ will offer (maybe good) games that came out in the last year or so, but just have them rotate out quicker. The rotation system is really key to how their system works, I guess.
It would make sense for Microsoft to do the same. Otherwise you're really only going to be able to offer really old games.
I guess Nintendo is releasing their Nintendo Direct video tomorrow morning. People are saying this could be the nail in the coffin for Microsoft but I don't really think so. Like, you're really into Smash Brothers and you're buying a WiiU or you're not. Nintendo's not going to shake anything up unless they showcase something no one has seen them do before.