I've stayed at both and WL wins for transportation, but not by much. But if you enjoy DTD like we do, SSR's proximity to DTD makes it very desirable.
As for the crowds, during that time of year is some of the absolute worst, which I'm sure you're aware of. It'll be more convenient to be at WL when going to MK but in reality, it won't matter much if you don't go there early. Think potential capacity closures on Christmas Eve and Christmas Day. So I would worry more about making sure you have plans to be in the parks early rather than where you're coming from.
I mean really, MK is good for WL, DTD for SSR, and this is largely because there are multiple ways to get from the resort to the destination. Everywhere else is only a difference of a few minutes, if anything.