This is my in-depth review after playing with every faction in the game, not only there are 6 factions to choose from, but there is also politics involved, I'll just point out the pros and cons of every faction so people looking at this can just play the game for fun or for political reasons. The opinion about being High command can vary, ranging from being a political patriot to a rogue traitor or spy. I'll write my long story short versions of each faction.
United Republic (North America): This faction, aimed at having good-various kinds of aircraft and artillery while being moderate in most departments.
European Alliance (All of Europe and Australia/New zealand): This faction, has average infantry but in return, very nice tanks and toys. This is also the biggest faction in the game player base wise, making their high command the hardest to get into.
Soviet Union: This faction has below average infantry, in return not only they get great walkers, but also one of the best aircraft in the game. The walkers require no deployment and can fire at the players will/desire, propoganda is also used to boost unit firepower :).
Shogun Empire (All of asia and most of the islands): This faction is one of the two infantry based factions, not only does it have paper tanks (except for the akuma class land battleship), it also specializes in being a fire happy/lover faction.
The Latin Junta (All of South and central America): The 2nd infantry based faction, unlike the shogun empire, junta vehicles are expensive and while some may be paper, other units not so much. This faction also specializes in toxic and chemical warfare, while they have good infantry, drawback is the infantry are frail.
African Warlords (All of africa and middle east): The beast faction, this soley focuses on being a beast specialist and improvised vehicles and weapons, even an improvised aircraft. The biggest thing that stands out here is the warlords "overlord" beast unit. If you like animals, this is the faction to join.
Conclusion: The faction units change (By the unit balancing department by the devs) every month or so. No matter who you join, each faction will have its pros and cons, as well as different communities (and political parties).
Can find me ingame by the name Aceman if you wish to reach me ^^