writing songs is the true test. Getting into playing fast, esp shredding, is like a bottomless pit of creativity loss. You become obsessed on a low level, and you get shred dismorphia where you never think you're playign as fast as somenoe else but in reality you are playing even faster.
I say...Music first, then shredding... I can't count the numbers of bands who have come to me for advice, and they are ALL super talented, except for in the songwriting domain... they can't write shit. In the end it's all about melody, yes even in death/speed metal.