IMHO Lem's work has a lot of "cyber" but no "punk" in it ;)
E.g. while Lem's future (in his science fiction books) isn't exactly a happy utopia, it's also not the depressing dystopia I would associate with "cyberpunk" - but maybe... "Peace on Earth" might qualify as "robopunk"? :)
Bester and Lem are amazing in their different ways, and it's tragic that both aren't better known.
I wouldn't put one over the other, but I think Bester gets the call because his view was classic mid-century US, and was an obvious influence on Cyberpunk.