in the 2nd case I’d rather choose USA and Japan as opponents.
both can be good, but the 2nd is a more balanced system i.e. probably better as a game. the best you can do is to support both 1) sieges and 2) open field battles, as in general in RTS games. for a demo I would use 2) to test the AI more easily, 1) would require 2 really different kind of AI strategies, and a more complex map decomposition system to realize key locations to attack/defend especially if the environment is fully destructible…