I would join Union,
But personally I like Confederates, because its was quite unfair, when Union before war led policy which were leading to developing industrialized South, and inner market, preventing South from selling their agro tradeship.
Whole war was made because of money: Industrialized Union were trying to make high taxes to prevent European- better and cheaper stuff- to develop they own industialization system. As response Europa started same taxes, what made tradeship very hard- South hadn't enough demand on its agriculture trades in USA itself, and made farming unprofitable.
Dixie tried to leave Union to save they states from bancrupt, but Union needed their "food". Both sides found in constitution against articles, one saying that secesion is posible (sth like each state can decide), and Union- impossible (to make global decision majority of states have to agree). Union started the war, but conflict raged on, when they revealed slavery (after waar begun!!!) as one of reasons, apart from moral side of this, confederate were concentrated about big plantations with huge amount of slaves, who had quite good life, they had food and "job". So it wasn't slavery so much as you can see on films, probably they had better life
there than they would have in Africa in that time. So without slaves whole structure of Dixie's would collapse. This are true reasons of war.