You need both in some form.

To sell the game and get it into the gamers hands you need good visuals in screenshots and Videos.

Once they have it you need good gameplay, and at least a few hours of it, to prevent them refunding it...

You also need marketing, otherwise no amount of gameplay or graphics means squat if nobody knows it exists...

what do gamers prefer, graphics over gameplay, or gameplay over graphics?

To be honest a lot of good games would still be good with bad graphics.

The main reason AAA games have good graphics is because it's become accepted and expected. People dont go out and drop £400 on a next gen console and expect pac man, they expect destiny etc. If one game has amazing graphics they all feel they need to match it or be ignored as "not as impressive" as their competitors. Advancing technology drives better graphics and therefore higher and higher art budgets and higher games pricing. It does not promote better gameplay in fact in my opinion quite the opposite, money is taken from the games budget and funneled into art development at the expense of other areas because graphics gives the first impression of the game on adverts, billboards, on the side of buses, and directly sells the game...

it seems that players look for a particular type of gameplay experience first, and then from titles with that gameplay, tend to choose the one with the best graphics, all other things being roughly equal. this means a game like dwarf fortress with better graphics ought to sell ok - once you account for the niche appeal of the game type, and the huge head start the original text mode version has against any newcomers. it also means that games like minecraft don't have to compete with call of duty graphics, because call of duty doesn't have minecraft gameplay. "best in class" seems to be where one wants to be with graphics - IE for games of a similar type, you want to be the title with the best graphics. I personally tend to use this to my advantage. i usually produce unique games with no direct competition, so whatever level of graphics i have is automatically "best in class", because its "the only one in the class". SIMTrek enjoyed this position for a number of years until Gene Roddenberry died, and Paramount entered the starship flight sim market. Caveman also enjoys this "only one in the class" advantage.

Dwarf Fortress is an exception, it has success despite terrible graphics and ui.

So.... if a game has terrible graphics and stunning gameplay, it's popular because it's an exception. I suppose if it has stunning graphics and terrible gameplay (nice view from this mountain top, I guess I better go to the cave, battle a couple dozen necromancers, and retrieve that book from some lazy NPC back in town for the zillionth time, then go back and find another identical quest), it's popular because it's a good game.

I see what you're doing there.