Is there going to be a choice like the choice to spawn as a Resistance or a Security?

When playing alone and with bots, you will join whatever faction you wish.

When playing with other people, you will join whatever faction you wish unless that unbalances the game through too many players.

In Campaign mode, you pick your faction in the beginning and stay in it while playing campaign mode.

In Freeplay mode, you pick the faction which has less players in the match (or not, but that is not cool).

In Stopwatch mode, you will play either faction once on every map.