Colour: The OB is at old gold (but remember Diageo uses caramel to fix the colour) while the BF is at jonquiripe corn. The texture of the BF shows medium fast legs and many small sticky tears. The OB shows much slower legs and late fat sticky tears so its oiliness is better.
Nose: The OB offers a real great nose on lots of different aromas with adorable peaty, fruity, malty and spicy wooden notes and some OBFs. It is very complex and beautiful balanced, one to sniff for hours. The BF is more closed due to the abv but owns a peaty, malty and wooden profile too, just the fruits are shy in the background. I like the OB more here.
Initial mouthfeel: The OB instantly coats all of the mouth with a warming feeling, no distracting moments (just as the texture promised). The BF is coating too (but less than the OB) and it has a slight drying feeling (metallic). Again I like the OB better.
Taste: The BF shows a peaty-barley sugary-herbal flavours profile that is somewhat layered and easy to sip. The OB is truly multi-layered and multi-flavoured on the palate, very delicious. I do not name all the different flavours but it adds nice chocolatey and honey-sweet notes to the peat and fruits. This is simply "lecker". I prefer the OB by several points.
Finish: Both own a long finish that nicely vanishes in waves. The OB is really sticky here and I still taste it even after minutes with all of its beauty. No bitter or drying notes at all. The BF adds more sugars to the finish and has no distracting moment either. Nevertheless I prefer the OB again.
Water opens the BF up and the nose gets more complex as does the taste - now it comes closer to the OB and guess what, I like it best when reduced to about the same strength as the OB. Water is not needed on the OB (it just gets flatter).
So this head-to-head showed a clear result: A batch of many (good) casks provide a greater aromas and flavours variety and a more balanced profile than just a single cask (of course, it all depends on which single cask was choosen). As all bottlings of the 1984 vintage of Caol Ila (that I tasted so far) were great the 2010 Diageo 25-year old should be and indeed is a stunner. I tasted all later 25-year Diageo Caol Ila releases and they are good too, but this one is the best so far.