In some ways the poll is hard to answer, lets say you use gmail a lot, is the OS of the client or the server the one you're really using?
It would be interesting to measure the usage of say posting this comment, how many machine instructions were used at the OSX end, and how many by the various routers (running their own operating systems), servers etc used in the full life cycle of what I am doing.
From my own experience, using Gmail is a different experience in each OS. I can't stand to use it on OS X, whereas on Windows it was always open.
(practically)