Why can't games be both? Like Team Fortress 2?
I normally like games that do things a little differently, so art style is important to me.
I mean, good art style can't save bad gameplay, look how poorly Velvet Assassin is being treated, but then again, Bioshock wouldn't have been anywhere near as engaging if it hadn't been a mix of Ayn Rand, Art Deco, genetic engineering, guns, magic and horror all in a dank, oppressive underwater setting.
Games like Silent Hill 2 are essentially all art style. Ico and Shadow of the Colossus and Katamari and The World Ends With You are all great examples of art direction taking a front seat to game play. And they're all great games, well remembered because of it.
I think a game with a unique art style stands a better chance of being remembered longer and more fondly than a game that uses a very trite art style.
Braid, Okami and Psychonauts will stand the test of time longer than all the WWII FPS that came out in the early part of this decade.