The not-so-simple answer is both.
The key to answering this completely is to identify the audience to which the material is directed. If your target audience consists of hyper-vigilant grammar police, then a misplaced comma will, at best, give them something to complain about, and, at worst, cause them to abandon the material altogether. If, on the other hand, the audience hungers for well-defined characters and intricate plotting, and the only thing that you can point to in your material is excellent craft in grammar, spelling, and sentence structure, the audience will move on to something that meets their needs.
Without knowing the specifics of target audience, there is no way to answer the question. If you understand the target audience, then you should already know what the answer is.