you can only compare based on players who have played in both comps..
NrL have better backs
SL has better forwards
I agree there's less of a gap in the forwards but George Burgess, Sam Burgess (prop in a LF's jumper) Warea-Hargreaves, Matt Scott, James Graham are probably the top 5 props in the world right now and all were in the NRL last year. I like Chris Hill but he's probably not at that level just yet, hopefully 4 Nations gives him a chance to prove he belongs there.
SL definitely produces some great talent, particularly forwards, but if the cream ends up in the NRL, it's another reason reason to rate that league above SL. This season they've had the best forwards and backs, even if a fair percentage are ex-SL players.