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Of course it's valid to consider the source, but you need to back it up, show that it's influencing his opinion unfairly. You may be right, maybe he is just favouring MS products, but then show why his comparison is wrong, or why he is ignoring better alternatives, something at least.

Ad hominem only describes the logical fallacy of saying "the argument X put forward by person P is wrong because of some property of person P"

You didn't make that sort of claim, therefore it really isn't an ad hominem attack

This might not be fair in dialectic, but I assure you that attacking the ethos of an opponent in rhetoric is fair game. It can be tricky if you only focus on that though because it does not make the audience believe more in your point... But of course, forget about this silly rhetoric thing, Hacker News is all about pure logic :-)