Having been a fan of both shows, here is my brief insight:
If you like discovering new voices and seeing the interaction between JYP, Hui-Yeol, and YG, I would recommend KPopStar.
It’s more of an American Idol-style show, but with the Kpop integration.
Produce 101 is more of a reality show style where you can see contestants grow through singing/dancing challenges, and there’s more of an audience participation part where it’s interesting if you’re following along.
Personally produce appeals to be more because KPopStar has been around for so long so it’s lost its initial hype (sort of like how American Idol was cancelled) but Produce is just like you can connect with these people and see how they improve and their personality.