So as some know, Bretons are apparently half human and half elf.
I look at the civil war as a small war between elf(Altmer) vs human(nord). So for RP purposes it could be a toss up. Maybe you want to provide the nords magic and to gain yourself a name within Skyrim considering, according to a certain dark elf, nords are racist. Maybe you want to support your elven pride and indirectly help the altmer by joining the empire to stop the rebellion.
To add a twist to everything I read that the forsworn are primarily, if not mostly, made up of Bretons. Now even though their isn't too much story behind the forsworn we know that Ulfric drove them out or Markarth? Something similar I can't remember but that could mean that you would rather help your fellow bretons or scared Ulfric might see you as a forsworn agent and deal with you some day.
so as a breton and knowing this information, what side of the civil war would you choose to fit your character. This isn't about which side is more fun, we're well beyond that by a year, but more RP friendly for your breton. Also feel free to correct me on anything. Been playing since Morrowind but never really dived into the lore very much even in Skyrim.