First, interesting idea to have fun together and tease one another.
Second, from reading online in other places, I think the hip style would be better, but I'm a guy, so I can't say for sure. Plus, you have better chances to hide it with most pants and other leg wear. As for the rear, I don't know, since there are many different things, but make sure you can clean properly.
If you do have punishments planned, look up accessories like thighbands or things to attach on the inside of the belt, it'll make your dares... interesting. 😏 But yeah. a bit TMI there, so I'll leave it at that.
Finally, if you are absolutely sure you want to try out a belt, take plenty of time to decide before buying. Get plenty of time to adjust to the belt, reshape, bend, etc, so you could wear it for extended periods of times. Most of all, welcome to the forum, I just saw some of your posts, so congrats on the trusting relationship and the shared secrets you'll be sharing, sounds like a joyful kind of play. (I guess you are the first official couple on here to use chastity as foreplay, if that's what you want to call it, but with both of you giving consent) Good luck and have fun!