Real thing, no way to get it to remember all the high scores, unless maybe someone has made a high-score chip, similar to these:
http://www.brasington.org/arcade/I imagine that mame would also have the same limitations. Of course, you could just never turn it off (I wouldn't do that with a real one, but why couldn't you do it on a PC? You could just leave BW on all the time and alt+tab to it when you wanted to play. That might not work so well with a front-end, though).
Chad, can't you wipe them somewhere in the test mode in Space Duel? I know you can do that in Tempest. I'm guessing ops would want to wipe really high scores on occasion because people like to think that they are good enough to post a "high" score. For years I thought I was good on Tempest because I could always get in the top ten (never could crack the top three, though). I wiped the old ones on my Tempest and now the top three scores all belong to me but would likely only annoy a future owner for the ten minutes it took him to beat them all.

I'm still no good at Tempest, but getting better...