I'd have to say offense is more difficult.
Like we've seen with Thibs and the Bulls, if you can get everyone on the team to buy into a system -- and I mean everyone -- you can put in Daequan Cook, for example, and trust that he's going to make the correct rotations, and play defense the way Luol Deng would (not at the same level, obviously).
When you look at offense, it's tougher because a lot of times teams have to make in-game adjustments on that end. If Carmelo doesn't have it going, Woodson has to be able to switch it up. Then you look at a team like the Spurs, where they've been able to create a foundation that allows Parker, Duncan, and Manu to still contribute even if they're having off-night. I guess offense becomes easier if you're given the time to let it grow and develop, while keeping your core around (Duncan and Pop, primarily).