I think you are right, you need to get a white-balance card - partly to simplify the target scene.
I don't have those camera bodies, so I am not sure what "on matrix metering ... and on the center point" means, but metering right on the edge of a white car hood and a green lawn may be emphasizing any WB algorithm differences in the camera models.
To me "matrix metering" means averaging over the frame and so I agree with you that it shouldn't make any difference exactly where the focus point is aimed . . . a white-balance card will give more clues.