This is a nonsense comparison as these features serve completely different purposes, while only having in common that advertisers currently use user tracking to achieve the same.

Topics data-mines your browsing history for information about your interests and reveals this information to advertisers in order to improve ad selection. It's meant to replace ad networks tracking each individual user's visits to connected websites and building that profile themselves. Since this is, in a way, much more powerful than tracking cookies, Chrome has a scary dialog asking for it to be enabled, and I don't think we'll be seeing it in Firefox. "Using different links" cannot replace user profiling at all.

PPA doesn't provide any new capabilities to advertisers. It's a privacy-preserving way of measuring ad campaign success that is currently done by ad networks tracking individual users from ad impressions to conversions. "Using different links" is also defective, as advertisers need to connect ad impressions to conversions even if they are not immediately connected through a click on the ad.

If these features become generally available, this reduces the leverage advertisers have on legislators to prevent tracking from being outlawed. Mozilla will be hoping Chrome picks up PPA.