What should a newbie know?

Hope it's okay to post as a non-union worker but union supporter!

I have been offered a new position, and it's a union position! It's in higher ed and it's a "professional administrators" union (edit: it's in Massachusetts, public institution). I'm terribly unhappy where I am, and this seems like a great fit. However, it's not a slam-dunk decision - salary is like $1.5K/year less (until I'm eligible for the increases under the contract next year), longer commute, and I miss out on the benefit of a paid week off between Christmas and January 2nd.

I'd been putting the union membership in my "pro" column for the new job, but I was wondering generally if there's anything I might be missing or misunderstanding. I'm super pro-union personally and politically, but because of what I do (pretty niche profession), I've never had the chance of membership. I know I'm probably being naive.

Thanks for any help!