Bennett E is where is damages are once you run it as your damage dealer. Phys as a secondary stat is just plain bad. This is even before considering that Bennet should be running crimson and want to boost his pyro abilities has much as possible.

Thus %crit or %crit dam is BiS on the sword to get as much as possible damage. (%crit damage being the best if you can be at 50~60% crit without the sword).

Base attack stacking get to diminishing returns as his Q just doubles (or more) it. The difference won't be that noticeable, as above 2k it's just not worth to get more atk anymore.

I have both sword at lvl80, and black sword out perform it by a more than significant margin due to far more consistent crits.

Note that if you can reach 60%+ crit without the black sword, then blackcliff should be your go to.

Bennett is pyro so stacking crit/crit dmg/EM is what you need to get as much dps out of him, at least as long as melt will be as OP.