This isn't easy for me to be impartial on....
I own a 1300MkII and have never failed to be impressed by its resillience, sound quality and recordings. I'm sure the 1500 will be equally good.
But I have been playing with 640H's for a while now (Cambridge are operated by the same owners as MS) and it really is extremely clever. It is best not to think of the units as being true competition. The Yamaha is the best (current) CD Recorder in the world whilst the Cambridge is the most price accessible audio grade music server. The flexibility that the 640H possesses over networks and when paired up together with the playlist organisation and OSD (the 1500's remains a bit primitive
AFAIK) it has considerable multiroom potential to boot. If you have the PC apps and hardware that will allow you to get the best out of the Cambridge it really is enormously more flexible.
ToF