Prefer Bronze Tiger for Vixen. In terms of building a franchise, it's always better to have complete editorial control over all major supporting characters and lore elements since comics are a volatile creative space where sudden changes in one corner can reverberate across several titles. That's the primary reason BP and Storm can never work out long term and why Mari and John would face similar issues. As of right now Vixen is operating in a globetrotting, Mission Impossible style space with a heavy dosage of the glitz and glamor befitting he status as a supermodel hero. John is spearheading the cosmic corner of the DCU in deep space. They exist under a completely different set of writers with different priorities as heroes.
And if they work in such different spheres you're limiting their ability to appear together in a meaningful capacity. It's the same problem you saw with Black Panther when he was filling in for Daredevil and you had a so-so appearance from Storm, who he was still married to. If they're gonna be dating, the expectation is gonna be it's not treated as some arbitrary add-on we see every few arcs in their solo titles. So the best way to get a Vixen/GL romance imo is to have them on the same team or to have them share a title where the main idea is they appear together every issue. Otherwise, they should just be casual friends who hookup between relationships every now and then.
Bronze Tiger offers a smaller profile character that can be absorbed into the Vixen franchise as well as someone that fits into her world of espionage and mysticism much better than John could ever hope to. He's worked with various intelligence organizations and has a similar level of global experience. He offers the same gruff, stoic personality of John that bounces off well with Mari's high energy. It wouldn't take much to reestablish their shared past in the Suicide Squad given how wonky DC continuity is nowadays either. I feel like overall he's a stronger character in terms of building a Vixen franchise with him as her Catwoman. A power couple with a cosmic character poses too many issues.