Depends on how you define "value" and "sexual marketplace".
If "sexual marketplace" is professional porn, then a lot. Some of the biggest names in porn had the biggest schwanzes. Almost all porn stars have at least bigger-than-average cocks. You hardly ever see actual average ones (five inch range), and in the porn world small ones don't exist at all.
If we're talking in the broader sense of the term, i.e., sexual attractiveness in everyday life, then it varies from woman to woman (or man to man in the gay world). There are, of course, size queens. Supposedly there are those who don't give a damn. In between are the vast majority for whom it matters to some degree but is not a killer issue, for whom other qualities act as a counterweight.
I'm hung like a chipmunk, but it's never stopped me from having a great sex life. Well, at least, overall. Okay, I've been shot down on occasion because of my size. Or, I suspect, not gotten a callback for that reason. But, generally, I've done fairly well, undoubtedly because of other things, like looks, confidence and personality.
You posited two guys, equal in all respects but cock size. I agree with
@Frankiefrom. "Everything is never equal." Cock size varies in importance from person to person, but it's very seldom the most important thing. Except, of course, to size queens. Most people place more importance on other aspects of sexual attractiveness, and those vary considerably. I've never understood why one person can make someone swoon, and another, seemingly very similar, can leave that same person cold.
There are a few exceptions. I think women (or gays) may seek a bigger cock for a one-night-stand or a brief fling, but settle for something less when it comes to a long-term relationship. After all, most of life is spent outside the bedroom. How many women wake up the next morning and wonder what they saw in the schmuck (besides a big cock). On the other hand, when I was single, I scored all the time, despite having a small cock. Of course, most women didn't know my size beforehand and were too far committed by the time they discovered my little secret.