Bah! I have to agree in part with SkyDancer, but I can see XArgos's concern.
I had the same choice working in local government ~30 years ago. I was asked to join NALGO. Under duress, I did, but made it clear that I wanted no part in trade union militancy of the kind that wrecked Britain in the 1970s. The lefties that persuaded me to join were right about one thing - the union did indeed secure pay rises in line with the 30% inflation of the day.
Unions can be OK, but they can abuse their power. Britain did OK though. We had Maggie Thatcher, and she absolutely smashed the unions.