Conventional Panerai collectorism would choose the 721 for the Kampschwimmer back story, less is more mantra, and on and on. It’s a nice watch, but unless its a real genuine Kampfschwimmer from the 1930s, I find the contemporary piece to be just a big empty watch. The Radiomir Panerai script of the 720 makes for a little more visual interest for me personally.
This might not help your Boutique sell a watch, but given that you have a 372, I would probably stay away from either of these and enjoy what you have. I was putting myself through the same paces at one time and realized that I don’t enjoy the Rad case as much as the Luminor case in the first place, so why should I own a watch so similar to what I already own?
Good luck with whatever you decide!