If you go for a bottle of Darkest, make sure that it's either very old (i.e. bottled at least six years ago), or make sure that it's very new (i.e. bottled in the latest run!)
The 17yo is a very different dram - not nearly as sweet or sherried, but a more austere peated dram.
I've done a head-to-head a few times....oddly enough, sometimes the 12 and 17 year olds don't seem all that different on the palate.