I'd supplement my earlier statement about "chords" (if you had to pick a "focus"), by saying that thinking in terms of chords will lend itself to useful "rule of thumb" rules, like hit a root/third/fifth for the chord in that bar on the 1 and/or 3 (assuming a 4/4 time). In my head, knowing that "in this bar, I'm aimed at the fifth of the ii-chord" is easier than thinking "in this bar, I'm aimed at the sixth of the scale." Obviously, knowing the key is pretty useful for determining things like "is the third of this chord major or minor?" . . . Which sorta leads me to thinking that thinking about the key could be slightly more helpful for writing and arranging, and chords might be more helpful for learning and improvising/jamming. Though, as some have noted, the relationship is so intertwined, and the OP's question phrasing leaves wiggle room for various assumptions and interpretations.
IMHO.
YMMV.
IMHO.
YMMV.