A good balance of both is ideal, but if I were to choose one I would take innovation ...
I think putting it in terms of movies makes it easier to understand. If you produce movies with the same basic plot line, with very similar characters, and the same art style all year (every year) how long is it before people stop going to see movies? Even if the quality of the visuals gets better and better, people will get bored of seeing the same thing over and over again and (except for a handful of the absolute best movies) most of these movies will be ignored by the audience.
We see this pattern in both videogames and movies today; and, if these games continue to get released ever 2 years, by the time Activision releases Modern Warfare 10 (no matter how good the game looks) people will have gotten so tired of the game that its sales will suffer in comparison to what the series current level of sales are.
Certainly, there are game series that have been around for decades and are still as strong as they ever were; but if you actually look at these titles you will see that most of them are based around gameplay mechanics that change from game to game, they tend to be released far less often than every other year, and there are few games that are successful that are similar to them.
