Depends if you're using it for exposure, or white balancing. For white balancing, it doesn't really matter so long as it's a reputable brand so that you can be sure it's neutral.
Exposure... there's will not be a massive difference between the two, but it depends how your camera's meter is calibrated. Sekonic light meters for example use 14% as the mid-grey reference.. Nikon apparently is 12... or 15.. depending if you believe Nikon USA or Nikon Japan.
Personally.. I'd buy either one and not worry. I've been using my Sekonic incident meter (apparently calibrated for 14%) for years with Nikon cameras, Canon cameras, Phase One cameras... you name it.... with no problems.