The two noteworthy aspects are: 1) if you click on an ad on startpage, you're inside Google's network. If you click on an ad on the duck, you're inside Bing's network. 2) the duck is independent and startpage is owned by an advertising company.

DDG is located in the US and logs partial IP information. Startpage is headquartered (and under legal jurisdiction) of The Netherlands. Also, it's a bit naive to call System1 just an advertising company. It's a bit larger of a company than that. Sadly, an ex-Startpage consultant did a lot work to confuse and manipulate users into not understanding their partnership.