Kevin, I already answered time ago to this question.
Mary goes to run New York. About Berlin, this is the project for 2013, when there is not a Marathon (Olympic) in Summer.
Florence Kiplagat goes toi run Sapporo HM.
You must understand that, at top level, the athletes cant chose what they prefer, always. There are offers from several organisers, there are technical and strategical reasons.
Under the economic point of view, the Defending Champion can have, in any Major marathon, a particular advantage, so it's clear that you can already know where the winners go next year. I suppose Geoffrey Mutai in Boston, Emmanuel Mutai in London, probably Mary can do the same, but, because the main goal is to win Olympic, can also happen that we chose to skip any marathon in Spring.
Everybody knows Berlin is the ideal Marathon if you want the WR. I looked at Berlin for Moses Mosop this year, but it was not possible because already there was some agreement with another top runner, that wanted to be mental free without having the pressure to fight with another Kenyan.
This is the professional Marathon World, and every decision depends on a lot of factors.
For amateurs runners, everything depends on their will, only.