This is a really tough call. Both whiskies are similar, with fruity grapes, melons and, perhaps, apples; chocolate, malt, and some vanilla. I'm not sure I get much sherry on either of them. I tend not to be good at picking sherry - especially in whiskies that mix sherry and American wood. But to my mind, Glenfiddich Solera Reserve has a more pronounced sherry feel and older Glenlivets are more sherried.
I confess that I tend to associate Glenlivet 18 with BMI airport lounges, so I am used to trying it in sub-optimal conditions. But nevertheless, the quality shines through. On the other hand, I have enjoyed a bottle of the Glenfiddich at home from a Glencairn glass. This makes comparison even harder.
I'd put both of these on a par, but I'd say that Glenlivet as a whisky has more in the tank - other expressions far exceed the 18yo - whereas Glenfiddich is getting about as good as it gets at the 18yo.
I must also declare some interest as I have received extraordinary hospitality at both distilleries during Spirit of Speyside festivals.