I guess you are thinking about the Viking "Mediterranean's Iconic Shores"?
It looks a really comprehensive intinerary, just about covers all the stops in the Med and then more!
So for that alone I think Celebrity would be hard pressed to offer anything matching even doing B2B's
Our experience with Viking Cruises was last fall in China, it was our first Viking. I have to admit I found it quite different
from Celebrity, the atmosphere on the ship is more "homely", obviously the ship is smaller, which contributes to this, but its
also the way you feel like you're treated as an "adult", very few announcements, you are expected to make your own way to catch your excursion, no nickel and diming, just makes for a tranquil feeling around the ship, just no hassle, which we really appreciated.
Entertainment was pretty low key, consistently good. The included excursions were mixed, some very good and others verging on mediocre.( having said that, this is more a reflection our chinese guides level of english not the content.)
The food (I know, very subjective) was consistently good to very good, generally a very pleasant dining experience. (probably edges the MDR on Celebrity but not quite at a Murano experience)
All in all we had a really good experience on Viking.
Looking at the cruise Barcelona to Istanbul I would be tempted especially for the itinerary alone.
It might be very worthwhile you looking at a Viking cruise as an alternative.