Everyone would prefer peace. But I understand your question, if war as inevitable, which side. I don't think it can be answered fairly, because we are looking at the question from the 21st Century perspective, with the luxury of hindsight. The world we live in today was not the world they lived in then. It is easy for me to say "The Union, the country must be preserved," from a 21st Century standpoint, and the fact I was born, raised, and live in Indiana.
Here is another question to ponder: If the Civil War was going to be fought today, not over slavery, but over current issues affecting our country, which side would you choose? That's a tough one.
I've often wondered about this question, precisely because I had ancestors who lived in the South and fought for the Confederacy, all for different reasons. One probably signed up for the $50 bonus, one was conscripted, one joined a month before he turned 18 and was probably just itching to get into the fight. And there were others whose motivations I'm not sure about. What would I have done if it was me? Probably tried to stay out of the war, but if push came to shove, I'm fighting for home and family and joining with the Confederate army.