Probably only one union will be accredited at your worksite, so if they had a table within the site, that will probably be the union that's available for you to join. Googling the name of that union, and visiting their website, should give you a better idea of how they work and whether they've ever been in the headlines for the wrong reasons.
I've been a union member for many years, and at one time had very helpful support from my union rep when a difficult situation arose, so I'm glad that I joined. But a lot depends on the particular union and the rep and the circumstances, I expect. Good luck.