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I'm not familiar with all brands/versions of polymer pistols made in both 9mm and .45ACP, but I've got two sets, S&W Shield in 9 and .45, and Beretta PX4 Storm in 9 and .45. In both cases, the .45 version is built bigger, so not apples to apples.

I have also had the S&W M&P in 9 and .45, and the Kahr CW9/CW45. Same thing, the .45 version is larger, so again, not apples to apples.

Unless you're thinking of a particular polymer framed 3" barrelled pistol whose 9mm and .45 versions are actually the exact same size, I'd say your question is fairly impossible to answer accurately.

My innacurate theory is that the 9mm would be shot more due to economics, so would wear out faster, though I've never worn out a polymer pistol. They all have replaceable parts.