They have some similarities but also are quite different. The 18 year old is a distillery bottling. It is a vatting of many casks and reduced in strength to 43% ABV. I believe it is also chill filtered and has some E150a caramel coloring added. It is still a very good whisky. The 21 year old is bottled by Signatory Vintage. They are an independent bottler that buys their casks from the distillery and manages their casks and stores them in their own warehouses. This particular Highland Park came from a single sherry butt, and is bottled at a cask strength of 55.6% ABV. It has some of the tell tale signs of Highland Park, so you get the heather honey notes that you experience from a distillery bottling, but this single cask expression is not chill filtered and has no coloring added. It basically goes from the cask to the bottle. Its higher strength results in more intense flavor, and even though it is a lot stronger I don't get a lot of alcohol burn from it so I drink it neat.
The aim with a distillery bottling is consistency. While there are some variations from batch to batch, vatting a large number of casks helps to control for that. Single cask expressions will typically vary more from cask to cask, but I feel there is greater reward to go along with the greater risk. When I first tasted the Signatory 21 year old Highland Park I was so impressed with it that I immediately ordered another three bottles while it was still available. Out of that batch of four bottles I finished one, am working on the second, and have two more in reserve. It will be a sad day when the last of it is gone, but on the bright side there will be other single cask Highland Parks to try.
--Jerome
