We bought the 617 pair back in December, and generally like them. As I recall, the higher models add more specific stain cycles and options, but generally have the same hardware otherwise. The 617 does add the 3rd pump which mixes detergent and water prior to entering the tub.

As for color, we picked white for the simple reason that they're cheaper. The gray is fairly dark, so it could make an otherwise small or dim space feel a bit dark, especially if on pedestals. Matched vs unmatched is probably a personal decision. Prior to this, we had unmatched Kenmores of different vintages for many years, and I don't recall ever being concerned about it. I suppose it could be a second reason to go with white, as you never know if they're going to subtly change the "other" color in a few years so it really doesn't match.