Subjectively it's the difference between "why am I getting ads for pregnancy tests and skirts, I'm a basement-dwelling troglodyte" and "wtf, they're actually showing me things I'd want to buy (if I had money)".

Joke's on them either way, they somehow haven't figured out I'm broke! But the difference is between "spying + garbage ads" and "spying + a bunch of cool shit I didn't even know existed" I'm gonna go for the latter.

Obviously the correct answer is neither: just use Adblock and/or pay for services you use and enjoy. For example I paid for YouTube premium so I could get an ad-free experience on mobile (because YouTube ads are somehow both horribly intrusive and horribly irrelevant, despite Google's apparent omniscience!).

By all accounts Google should know much more about me than Facebook does, but somehow their ads invoke a response somewhere between mild irritation and outright rage and disgust. Meanwhile on Instagram: "hey, I really like this backpack", "wtf they're selling psilocybin in capsules now? And I can just buy it? Nobody even told me that existed! Thanks Instagram!" Like I said, that was a pretty surreal moment for me.