The standard Glenfarclas releases (i.e. 10, 12, 15, 21, etc) are not sherry monsters. Yes, they are certainly sherried, but I do not believe they are exclusively so. (In other words, I understand there is some bourbon in the vattings, although I couldn't say for certain how much).
For me, the Glenfarclas 15yo most closely resembles the standard Macallan, although if comparing directly, I'd say the Glenfarclas offers just a little more spice.
Needless to say, I think all the Glenfarcli are great bottlings, and I've never been disappointed with any bottlings - which is more than I can say for the odd Macallan.
For a real sherry monster, you need to get a single cask - one that was a great cask to begin with, and one that treasured its contents for at least 15 years or so. The most amazing sherry monsters I've tried were single cask bottlings of Glenfarclas (from a 30yo butt), an 18yo Glen Grant from a hogshead, and a 24yo Macallan from a butt. The sherry was extraordinary, but they were all sensational, stellar drams.
Frodo - my advice, for what its worth, is to first try the 15yo if you can. Not that there's anything wrong with the 21yo, but the extra 6 years in wood has taken a tiny sting out of the fruit. But in truth, buy any Glenfarclas....you won't be disappointed.