I'd say getting the gameplay core loop done should be first priority, make sure it's fun in it's simplest form, then art would be fine to work on. Having a good looking game that feels clunky and unfun is often worse than a simple looking game that is fun to play.
Also if you have amazing art at the first round, but not a lot of content, then anyone who adds content would have to either match your art, or scrap all art assets to make it look consistent. And if they aren't as good of artists as you they are more likely to choose the second option.