I like both. Greenfield projects can be pretty boring when there isn't something special about the functionality or a new architecture to try out, but it feels good to spurt out stable features when you know what you are doing.

Repairing / fixing / retrofitting an old code base always has it's wonders. I've seen much code in my life that was in a bad shape and my approach for fixing was seldomly the same, it was always tailored to the team and situation.

To rephrase from Tolstoy: "Good code bases are all alike, every bad code base is bad in it's own way."