In all honesty, I can't tell the difference between them, and I don't care that much, either. I bet if Arial had come first and Helvetica had been derived from it, people would be all down on Helvetica. All this business about which is to be the "authentic" generic bland sans-serif is completely uninteresting. Neither one is particularly sexy, and certainly I don't believe either one is "optimal" in any sense. If you're a professional type designer (which means you are probably about ten million times more finicky and time-free-for-type-design-having than me) why not keep tweaking around in that region of design space until you get something more attractive instead of whining about it? Or better yet some other region entirely, but note that this activity still takes the place of whining.