You seem to be ignoring the fact that in both the total appearances and minutes played stats, Martial and Rashford compare poorly with most of the other players. I don’t see therefore how this is silly, when it is factual.
I'm not ignoring it, because the "fact" is clearly affected by the fact that Martial has played one season exclusively as a striker, and Rashford maybe half a season - his very first one. It's also useless to look at their aggregate stats going back to when both were teenagers and clearly inconsistent. Martial's goals-to-minutes ratio has been at or above 1 in 2 games for three seasons now, which is reasonable enough
Today, if we wanted to strengthen our attack and they were playing elsewhere, would you buy one or both and if so, how much would you pay?
I don't think this is quite the gotcha question you seem to be framing it as.
If Rashford and Martial had hit 17 league goals each playing for another team last season and I wanted to strengthen United's attack, I'd obviously be looking to buy both of them. Rashford is in the four or five best left-sided forwards in the league right now, and the only strikers I'd say are clearly better than Martial (assuming he isn't permanently crap now - fair assumption for me considering he's hardly the only player in the league struggling for form this season) are Kane, Aguero, Vardy and Aubameyang. Werner probably will be once Lampard gets a clue or Chelsea bring in a good manager.
I never said they're top drawer or beyond reproach either - obviously, you want someone at the very top bracket and neither of them is quite there yet. Last season was the first time both of them played to a level you'd want for a team that wants to compete at the top. Rashford is building on it now, Martial isn't, both of them should be doing better, neither of them are world class.
But they're both good players who've been getting some outrageous criticism (Rashford in particular) from the legions of knee-jerk geniuses on this board. Your comparison with Van Nistelrooy wasn't nearly as god-awful as that Dan James one, but it wasn't particularly illustrative either.