I honestly don't think you're going to approach, much less reach, a resolution on this question. Not in a science-supported kind of way.
If you had to make a pure choice between the two, and completely eschew the other, clearly Gameplay is the way to go. Gameplay at least has the potential to stand on its own as a game (seems obvious), but graphics can't. Graphics support gameplay, not the other way around.
But that's not a realistic scenario. You'll spend time and energy developing both gameplay and graphics, and in the end you'll still probably fall short of everything you wanted to deliver. Many of your customers' or potential customers' first impression of your game will be made on aesthetics -- the style and quality of the artwork. If that draws them in, they'll seek to learn more about your game. First impressions are important, but they're not sustaining -- in some sense, the statement "I came for the graphics, but stayed for the game-play." captures that. Still, that's not everyone either -- I can't imagine many folks are impressed with screenshots of Dwarf Fortress, yet its got a good cache of hard-core followers. People simply have different and complex preferences, there's no single voice.
For me, I'm perfectly happy to enjoy simple graphics -- I prefer for them to be 'clean' and well-polished for what they are, but I'm willing to overlook imperfection as long as they are not aggressively bad. And, for me, no amount of stellar graphics will make me want to play a game that I don't find any fun. I bought Gears of War day one, easily one of the best looking games of its day, but I didn't like how it played. I've still got it, its been years, and I literally haven't touched it since -- and lots of people loved that game, but I didn't. On the flip side, I still play (and buy) games for all my classic systems -- and its not pure nostalgia because many of the things I buy are things I never played in their time. Likewise, I'm perfectly happy to pay new game prices for retro-looking games (Why not, I pay collectors' prices for most of the vintage stuff).