IMO, I think varying it between both is the best. You have to mix it up to keep your body guessing. Also, it used to be a golden rule to lift VERY heavy but at minimum 8 reps to build more muscle. I haven't seen anything different from that but maybe there is newer research saying differently.
On second thought, when I reread your post, the second option sounds like it would be the best way to work your muscles to failure.
Marla