I have no dog in the fight, but if I had to choose, I'd pick the Bengals.
I know people in LA who were big Rams fans…until they skipped town and went to St. Louis.
While gone, they lost an entire generation of fans, who flocked to other in-state teams, like the Chargers, Raiders, and yes, 49ers. Or the semi-pro college team in town
The diehards won't forget being jilted. Some who never developed any attachment might jump on the bandwagon, and that's a common thing anyway during the SB. With the game in LA, and the Rams playing, the fair-weather fans will probably reach a high, especially the celebrity types.
A SB win will mean a lot more to the folks in Cincy, and for that alone, makes it the team easier to root for.