The biggest difference between SPDY and Microsoft's proposal is that Microsoft's proposal is only theory at this point. The SPDY proposal has dozens of independent implementations behind it and 3 years of operational experience.
I'm not trying to dis the proposal; just point out that it is in its infancy. To jump start it, Microsoft started with SPDY at its core, but changed the syntax.
Can someone explain how Microsoft's S+M proposal differs and how well it would work in the real world with proxies, firewalls, NAT etc? (I'm being lazy and haven't read the paper)