What I'd love to do is tie a Kobo promo with a big promo I'm already running. Like imagine one of their book feature ads while running a Bookbub promotion (anyone of you ever do that?). For some reason, when I try to pair them they always reject me, even when I apply to more standard free promo applications I've been accepted to before. Murphy's law.
You want to move up on their promotion carousel. Ads can help with this. When your book shows on the carousel, it sells. When it doesn't, it gets only one or two sales during the promotion, ime.
As far as paid versus not, the paid $10 free promotions can give you some visibility. But be careful. If you run a free promo on Kobo, your book might be price matched free on Amazon.
As far as whether the paid are worth it ... they're affordable. You might get a limited tail.