I am posting this here instead of the theory threads because I believe it has more to do with notion of concept and not actual music theory.
I have found that for me I can learn material way faster than if I sit down and try to write something from scratch.
Here is the part that is my main concern. I can write things out and structure riffs into a song format, then learn/memorize the song easy peasy.
Though, just playing pure awesomeness in an improv situation isnt going to happen.... I either let whole notes ring, or default back to pentatonics, aprs and scales.
What are you better at? How do you mush these subjects together?
Honestly... writing just takes time and practice. When you first started playing guitar you couldn't play scales or arpeggios, same goes for song writing. It takes practice.
That said, you can learn to write good songs. At its heart, metal, rock, etc. is all pop music. There are basic structures that work, and melodic ideas that work, etc.
Read here: The Elements of a Great Pop Song - Gearslutz.com
At the end there are a couple of books mentioned...