Sorry, can't help from personal experience. I've only tasted the 10yr Auchentoshan, and I didn't care for it too much. Its pretty surprising that you would go from an Islay whisky (albeit, Bunnahabhain, which is a light Islay) to a Lowland, known for its triple distillation and very "thin" profile.
However, the 21yr from Auchentoshan is supposed to be quite the dram, with a dramatically different flavor profile than I would expect. Here's what I was able to find about it:
Originally Posted by lincoln imp from whiskywhiskywhisky.com
Colour - dark amber
Nose - Vanilla, marzipan, honey, banana
Mouth - Spicey, more vanilla, marzipan, slight orange zest
Finish - Medium length, warming, gentle on the palate.