San Francisco 49ers over Cincinnati Bengals
This is about as easy as it comes. The Cincinnati Bengals are not playing a team that is in the AFC playoff picture so there is no "lesser of two evils" conversation that we have to have about which AFC team we should be rooting for. This is an AFC playoff contender against an NFC playoff contender.
The 49ers winning this game has absolutely no impact on the Chargers while the Bengals losing this game would have an impact. It would not move the Chargers up in the seeding at all, as they are already the fifth seed in the conference, but it would give the Bolts some cushion as they head into the final four games of the season.
The Bengals and Chargers currently both have a 7-5 record with the Chargers having the tiebreaker advantage over the Bengals. Cincy losing to the 49ers would essentially give the Chargers a two-game lead (assuming they beat the Giants) as they would have a one-game lead with the tiebreaker. A
Adding another team to the mix at six losses only helps the Chargers. There are a slew of six-loss teams currently in the conference and there is a good chance that a few of them lose. The more those teams pile on losses and the more the Chargers win the less we have to talk about the Bolts missing the playoffs altogether.
To be completely honest, the difference between the fifth seed and seventh seed probably isn't much with how wonky the AFC is this year but the quicker the Chargers can clinch a playoff berth the better.