The best Sheratons are better than the worst Westins. There are a handful of domestic USA Sheratons that are very nice hotels, but most are in the "acceptable but nothing special" category. Many international Sheratons are nicer than domestic USA Westins.

But really, the big difference is that they were two completely separate companies until the formation of Starwood, and they were both very strong brands, neither of which was worth losing. "Just what to do with Sheraton" and where it fit in the Starwood mix" seems to have been an issue for some time, and it looks like Sheraton is finally finding its way.