Good For You, though I do love Cool For The Summer's chorus. The poll will always swing towards the more recent song so I don't think a comparison's necessary right now (or ever, honestly, they're different artists with completely different songs! They can both be great!)
I have to be honest here...I've always liked Selena and disliked Demi but...Cool For The Summer wins it for me tbh. It's explosive and hot. GFY is still good but it had to grow on me a little first while CFTS was instant.