I'd recommend the 6D over the 5D MkII, due to the newer sensor/processor technology. The thing that separates the 5D MkIII from the 6D (besides obvious build quality) is the MkIII's superior AI Servo mode AF system. Most people really don't need that, but if you shoot lots of sports or birds and wildlife, then you'll need to consider it seriously.
Looking on Flickr, I see some excellent files produced by the 6D. For most people, it's all you'll need and it'll be a huge upgrade from your Rebel. Buy the 6D and focus on buying quality lenses, which, BTW, includes AF lenses and zoom lenses.