I shoot mostly B&W and have Ektars, newer Schneiders and a Dagor. there is differences between contrast, but quite a bit of this can be controlled with proper lens shading and filtering. With color there is a noticeble difference in contrast and color saturation between say the Ektars and Dagor compared to the new Schneiders. So if you want consistency in color one mfg or maybe stick to very current lenses.
I suppose if I had the money I would upgrade to all Nikors or Schneiders, but every time I think about buying a new Symmar L lens I think about how I can buy a couple of older lenses for the same amount.