I suspect that in the long term things like IS, focus accuracy, feel in the hand, balance on the tripod, control feel and location, and other basically mechanical issues would have more impact on image quality than sensor issues. A clumsy, flailing camera/lens combo will produce bad pictures no matter how good the sensor is.
Just hefting the two cameras with the same lens might give you 90% of a practical answer. Easy to arrange if you live in a very big city, but otherwise not.