The "donut" bokeh you are seeing in the viewfinder is probably related to the way the reflex mirror is made in modern DSLRs. The center of the mirror is not as reflective as the outer edges are. This is to allow some of the light in the middle to pass through the primary mirror and bounce off the secondary mirror into the Phase Detection Auto Focus array located in the floor of the light box.
The widest parts of bokeh come from the widest parts of the lens. The light rays coming through the center of the lens are more collimated and produce less bokeh. This is why stopping a lens down reduces the amount of bokeh. So if those collimated rays are reduced in intensity (or even blocked, such as with a mirror lens), the bokeh will be brighter on the edges (where the mirror is more reflective because the outer parts aren't translucent), than in the center.
Don't forget that what you see through the viewfinder is almost always seen with the lens wide open, regardless of the camera's aperture setting. The lens is only stopped down the instant before the shutter opens, usually while the mirror is being moved up out of the way.
The reason you are noticing it now, with your new 6D, more than with previous cameras could be due to one of several things. It could be a combination of all of them. Possibilities include a difference in the focusing screen, reduced transmissibility of the center of the reflex mirror, or the slightly larger viewfinder that may also be slightly brighter (to make up for the lower reflectivity of the center of the mirror).