Well, they are very different (not sure if I've had the most recent 15yo - it's been at least a couple of years !).
I cannot remember the 15yo very well other than after sampling a friends bottle, I would pass on it at the price. The 1997 is very sweet and powerful, the extra charring is apparent both in the colour and taste. I'm a regular bourbon drinker and those charred toffee notes reminded me of my 'other' favourite tipple. I'm not saying it tasted of bourbon, because the sherry is too apparent. Neat Vimto squash maybe ?
I nearly bought a bottle and I'm flat broke at the moment (I got a free sample in a shop - one enthusiast to another kind of thing
I've put it in my PDA's to buy list along with a CS Benromach and Rosebank Jim Murray raved about .....