Definitely composition. As a long time guitar player, I've been composing loops since I was 12, but never really mastered writing a complete work where the parts fit together well and take the listener on an interesting musical journey. It will also speed up my production process if I have a solid framework for the composition and arrangement before diving into sound design and mixing.
Of course, I'd also like to understand the fundamentals of how to make a project sound more professional, but there are so many people releasing reasonably well mixed, self produced music that it seems it must be a lot simpler than it seems once you understand the basics. Not everything has to have Noisia levels of fidelity to be competent but I've found that even if you have a decently composed work, if it doesnt compare to what people are used to listening to, it is very hard to get people to listen to your music all the way through.
