Just to address the elephant in the room - besides both being in Scotland, Islay and Kelso are nowhere near each other! But I happened to visit both in the same week just after our wedding, so that's why they're lumped together here.
Islay is a beautiful island, but doesn't really do it for me personally in terms of the scenery. I'm more a lakes, trees and mountains kind of guy - Islay is more of the flat and featureless vibe. The drive up there was incredible though, up through the Rest and Be Thankful, and I enjoyed touring the Lagavulin Distillery.