For this argument, is there a meaningful difference between a PBC and an LLC?

There are many meaningful differences, from corporate structure to ownership and tax classifications. For example, LLCs cannot issue private stock to shareholders. PBCs are effectively C corporations[0], but with bylaws that include making a positive impact on society (not just maximizing profit)[1].

[0] https://en.wikipedia.org/wiki/C_corporation

[1] https://en.wikipedia.org/wiki/Benefit_corporation