Of course there's a difference. Instead of hundreds of advertising companies having access to my data, this way there's only one.
And that's even ignoring the difference between my ad preference data being stored on a company's servers, versus stored locally on my machine by my browser.
I like your optimism, and I don't mean to patronise you, but I agree with your points if I keep the short term in mind. In the long run; however, I feel one would be disappointed in what the ad-industry will turn this privacy-sandbox into. Refusal to ack DNT and dark patterns in-use all throughout the interwebs stands testament to this.
Advertisement has, unfortunately, degraded into a plague spreading scareware, spyware, malware, scamware and what not. It gets worse with each passing day as more unsuspecting users discover internet for the first time. The surveillance appartus that the industry has built has made robots out of humans, mere instruments to be toyed with. Not all parts of the advertisement industry is worthy of our data but have access to it anyway, because they've got the money to burn that companies like Google and Facebook scramble over to gather.
These companies need to be forced to come up with alternative business models or ads that do not require extensive surveillance, or I am afraid given the unprecedented data they have amassed, they will attract, in the long term, all sorts of power-hungry designs and the outcome is going to be a disaster for everyone else but the power-grabbers. The result isn't going to be pretty. Sorry to go all Orwellian on you.
> These companies need to be forced to come up with alternative business models or ads that do not require extensive surveillance
That's exactly what Google is trying to do here. They're replacing the existing ad system with one that does not require surveillance to achieve ad targeting.
And that's even ignoring the difference between my ad preference data being stored on a company's servers, versus stored locally on my machine by my browser.