Backlund definitely has the higher ceiling, bigger fantasy impact and should be the better offensive option.
Backlund has been brought along slowly and when he makes the NHL (he'll see some time this season again) he will be put into a top 6 role.
Benn could also be a top 6 player but I see him plugging his way as a 3rd liner in Dallas until other vets start to fade away. He has more competition IMO
Backlund Upside 80-90 Low End 60
Benn Upside 75 Low End 40-45