I will say both

, but that is because I'm inside the sausage factory and have a very different perspective than most.
For the most part the fan art is better than the Firaxis or Breakaway art (sorry guys). This is not due to skill, rather this is because a fan can spend as long as he needs to get things right, to his taste. Firaxis and Breakaway artists were working under deadlines, as well as budgetary and artistic constraints. I know I spent more time on my terrain mod than I'd ever pay me to work on it. On the other hand, the content that came with the game is very tight; especially considering the time that they had to do it in.
I do not like most of the fan scenarios. They almost all suffer from the kitchen-sink approach to game design.
More is not better. It is almost always worse. It leads to a jumbled design that isn't remotely balanced, paced, or in a lot of cases even strategic (sorry Martin, you put too much in there). The Breakaway-made Conquests are much better, as they are mostly tight scenarios (and I'm not saying that to suck up to you, Ed).
One exception is the quality of the fan maps (not the scenarios that are built upon them, the maps themselves). These are mostly very good. Having said that, the ones in C3C were pretty good (the main bad ones were the world maps that shipped with the vanilla game).
One important thing to note, the biggest fan mods are used by less than one percent of the Civ playing population.

The rest of the people only use the stuff that is included on the disk. (The exception, of course, is the fan-created content that is included on the disks). This makes your poll very biased to start with by even putting it on this site.