Colour: Both have the same colour of old gold (yes, Diageo uses caramel to fix it that way). The texture of the 25y shows some fast fat legs and late sticky tears. The 30y shows early but very small tears and some late slow legs and fat tears.
Nose: The 25y offers this adorable sweet and strong phenolic profile with maritime aromas and some nice OBFs that I adore, one to sniff for hours. The nose of the 30y is more subtle but on the same profile. Hard to say which one is better - the 30y is more complex and delicate while the 25y is more powerful and impressive. Finally I opt for the 30y.
Mouthfeel: The 30y instantly coats the whole of the mouth with a slightly peppery feeling but without any distracting (bitter or drying) moments. That is really good! Surprisingly the 25y is smoother than the 30y on the mouthfeel but at least as much coating.
Taste: A very mellow and complex sweet-peaty-phenolic-maritime profile on the 25y that is multi-layered, really excellent. The 30y counters this with more punchiness (surprisingly) and a greater complexity of flavours adding more spicy and malty aromas to it than the 25y. But again I like the 25y a little better (on a very high level).
Finish: Both are very long with the 30y vanishing in several waves without any distracting notes. Close to perfection it is (remember Yota)! The 25y finishes a little shorter (just long, not very long...), but again without any flaws or off-notes. This round wins the 30y year old.
Water is not needed on both (it does not really harm and smoothen the drams even further, but it takes away some power, too). I am really blown away by the high quality of both drams, the only flaw is the drinker-unfriendly price tag choosen by Diageo for the 30 year old. I am willing to pay as much as 250 Euro for the 30 year old - but not a single Cent more!