I've only had the Glenlivit and Glenfiddich in the earlier ages (ie. 12 year old) and found them to be similar (both are speysides). I like them both, but would lean towards the glenfiddich as I feel it has a little more substance/character to it. I've heard some good things though about a glenfiddich 15 (there are differnt types of 15 year olds).
Note: I did some research and if you have the Glenfiddich Solera Reserve 15, you apparently have one of the best Glenfiddich's around...and a great whiskey.
Smitty, if he's up there in age, he may be old fashioned in his whiskey drinking and stick to the old stand bye's (ie. Glenlivit and Glenfiddich). If this is the case, maybe go with something he's familiar with, but in an order age that he wouldn't usually buy, and would then recognize as a special gift. I did a search in Edmonton for the Glenfiddich 15 Solera Reserve, and it's in many liquor stores...go to superstore for the best price.
I usually don't drink Glenfiddich, but may have to pick this one up. Just another idea for you Smitty to confuse you even more