I think they would stay at Google (in its current state). Google is getting big but is still not as big as Microsoft (political). Google has more data (I would imagine that is something search folks want more of). Google seems to be the best environment for top search engineers for a variety of reason. My guess is that the top search folks (along with others in the top of their field) get to choose where they work and what they get paid because they are at the top of their game. So if Microsoft is where they wanted to be they would already be here. If it is where they want to be and they are not shame on us for not recognizing that.