Unions are cool if you're planning on being an MT or an MA for a while. While going to college I worked at a supermarket which was unionized. I worked there for five years, and there are a few things you need to keep in mind.

Unions work on senority to get promoted which kind of sucks when you want to move to a different position and someone out bids you because they were hired before you. Also many unions have not only monthly dues but a one time initation fee, I think mine ten years ago for a bagger was like $120.00 plus there's a new fee everytime you change classifications. If you don't mind being an MT or an MA for awhile then the union is the way to go, but for a position for an MT you are really supposed to get promoted as fast as possible.

Once you're in managment you're out of the union. Also if they call a strike be prepared for no paychecks for the duration of the strike. in supermarkets many people kept a strike fund going. Not too sure if any MTs could swing a month with out pay. I know I couldn't.