No confirmed mechanical difference exists between Lonestar, Valkyra and Manticore. No source shows faction-exclusive weapons, vehicles, starting cash or progression. The one confirmed difference is cosmetic: each faction has its own free starter rifle. Pick whichever side your friends are on.
Your pick is not permanent. Press Escape and choose CHANGE TEAM. It works mid-match if the faction you want has open slots. If it is full, wait for players to leave.
BULKHEAD's FAQ describes teams as drop-in, drop-out and says one extra player on a team does not give it an unfair edge.