I have a slight preference for allowing both players to use the same army, but it's fine if we don't.
In addition to the two army-selection methods already proposed (sequential choice and random assignment), there is also the possibility of secret or simultaneous choice. If identical armies are forbidden, there is still the possibility that both players may choose the same army; in this case I like Ralph Betza's suggestion of letting Black choose which player will change armies.
I would be content with any of the three methods, so long as any advantage from the army-selection process goes to Black. In particular, if we choose sequential choice with identical armies permitted, then Black should have the second choice as Greg has proposed. If we choose sequential choice with identical armies forbidden, I think Black should have the option of choosing first (so as to be sure to get his favorite army) or second (so as to have maximum information available when he chooses).
Also, the order of choosing can make a difference even if identical armies are permitted. A player's preference may depend on which army he will be facing. Random example: If Alice is playing the Nutty Knights, then Bob prefers to play the Colorbound Clobberers, but if Alice is playing the Fabulous FIDEs, then Bob prefers the Remarkable Rookies.
In addition to the two army-selection methods already proposed (sequential choice and random assignment), there is also the possibility of secret or simultaneous choice. If identical armies are forbidden, there is still the possibility that both players may choose the same army; in this case I like Ralph Betza's suggestion of letting Black choose which player will change armies.
I would be content with any of the three methods, so long as any advantage from the army-selection process goes to Black. In particular, if we choose sequential choice with identical armies permitted, then Black should have the second choice as Greg has proposed. If we choose sequential choice with identical armies forbidden, I think Black should have the option of choosing first (so as to be sure to get his favorite army) or second (so as to have maximum information available when he chooses).
Also, the order of choosing can make a difference even if identical armies are permitted. A player's preference may depend on which army he will be facing. Random example: If Alice is playing the Nutty Knights, then Bob prefers to play the Colorbound Clobberers, but if Alice is playing the Fabulous FIDEs, then Bob prefers the Remarkable Rookies.