Is there any advantage to joining one team over another?

I don't think there's any king of advantage or disadvantage in joining a particular team. It would only kill the fair play which the developers of the game would want the least. Joining a team show more about which legendary Pokemon you like. Each team has it's own Pokemon Mascot.

I think its most likely just ideological and cosmetic for the time being, perhaps in a future update, I find 10k eggs fairly often, I hatch big pokemon, and my pokemon level up to be just as strong as equivalent ones on other teams..maybe not exact, because I believe there is a base stat range on pokemon you get, like I had two vaporeon that were 10 cp apart, and every level up gives over 30...so the only thing I can figure, is that the base stats had to have been different, so it could be a coincidental difference in strength regarding similar pokemon powers