Haven't played since release.. is there any difference in the three teams you pick?

No mechanical difference between the teams has been introduced. It's just a question of do you want to look like you got your tongue stuck to a pole in winter, that you fell asleep in the sun or that a horse peed on you.

Only difference is the local distribution of players. Most areas have a team that is more dominant and/or aggressive.

If you are on the dominant team, you will have competition for getting into gyms, and less choices when looking for gyms to fight, but will be more likely to get team bonuses (extra items when spinning, +2 premier balls for raids).