I'll start the ball rolling by saying that there may be a fair number of folks who, like me, have not come across StartPage before, whereas the great majority will at least know of DuckDuckGo. So comparisons of actual usage and experience may be limited.
I have a somewhat ambivalent view of "Search Tracking". I dislike the principle, but live with the actuality for most of my searches - I use Google. It's fast and usually very reliable.
Where I consider something to be more sensitive, I use DuckDuckGo - and if even more sensitive, I use DuckDuckGo via Opera and its free VPN.
I don't care very much for the DuckDuckGo UI, but what the heck, it's a search engine. It is not as reliable, in my hands anyway, when it comes to searching for scientific and medical articles which occupy a lot of my time.
That's my meagre contribution until I research and review the possibilities that StartPage might have to offer.
Ian