For the vast majority of the game's audience, BCE would be more immersive.
This is a game. It's an application to be used by modern humans as a window into the past. BCE, by using the calendar system they are actually familiar with, with serve to remind players that they are playing around in game that is supposed to represent actual history.
99% of those modern humans will have no understanding of AUC and will not really care very much about it. It won't feel immersive; it will just feel arbitrary and abstract. For most players, you might as well use "X years since campaign began" if you're using a non-BCE dating system.
And using AUC is really just more misleading than immersive. Making it the main dating system suggests that it was used earlier, regularly and more widely than it really was. You're not improving immersion. You're just misinforming players and making the game seem less grounded in history at the same time.
Nobody (besides apparently DreadLindwyrm) would look at BCE and think it was used for any reason other than it's the calendar system used by the audience.