Gameplay for me, by a large margin.
It's what I play games for. If I want an engrossing story, I'll read a book or watch a movie, but I will never go for a game that focuses on the story far more than gameplay.
If there's a balance of fantastic gameplay and a fantastic story, then great, but unfortunately that is very, very rare. So many games these days, at least in my view, have a decent narrative but mediocre gameplay. It makes me somewhat sad that so many developers are shooting for the cinematic experience such as this. When games have cutscenes for longer than a few minutes, it just gets on my nerves.
If the gameplay is fun and awesome, however, I'd happily play a game that's just about a man finding his shoes. It's all about the gameplay and how fun it is.