Absolutely none whatsoever :) Benjamin
They cost more, but they last longer because there is more stone there... The 8k it is not even a consideration, my 4/8 has done a LOT of razors probably close to 3000 the 4k side is getting down there but still has many more razors to go... The 8k side I don't think has more than 1/16th of an inch gone from new...
I had promised this stone to a friend, when I decided to buy a new set, this adventure just prompted me to make the move, early and I had a gift card to Woodcrafter :) which just happened to have them in, and I just happened to be in Spokane anyway yesterday. :p Karma
But no I don't see any obvious advantages, just my mileage varied toward separate stones YMMV