BB Rows or Assisted Chins? Well, I would say that BB Rows are a better exercise than assisted chins, but having said that, I would still incorporate assisted chins into your routine as they're both different planes of movement, horizontal / vertical. You could cycle between both exercises every 8/12 weeks (carry on with assisted until you can do them unassisted as mb states).
When I was doing 5x5 deadlifts I wasn't keen on doing barbell bent over rows after as my lower back was pretty hammered from the deadlifts. Other people have a different view and find that they're fine with this. I prefered either to do supported DB rows (or, of course, chins).