The IBM and/or the SAM are both worth joining.
If you can only join one, or only want to join one, find out which one has monthly meetings most convenient for you.
If they both do, then go to both a couple of times and see which one you like the best.
Who knows? You may end up joining both, like me!
The annual membership in each includes a monthly magazine and the dues are less than many magazines charge for a subscription alone!
For more information:
My preference? Well, I'm a bit biased in favor of the IBM and its magazine the Linking Ring, since I've been writing a monthly column there for more than 10 years.
For other organizations, check them out on a search engine like Google.
The same with magic magazines; there are zillions, depending on your interests -- comedy, bizarre, escapes, mentalism, general, kids, etc.