SOSA is the way to go. We always had the best food at meetings, at least at my school haha. Honestly unless you are doing leadership at your school or nationally, these memberships don't really do much for you. Most students join clubs for some perk or to pad their CV. At my school, UAAO has practice practicals for OMM, SOSA has practice anatomy lab practicals, and so forth. I pretty much signed up for every club at my school but only actively participated in SOSA and military medical club because I was strongly interested in surgery and military medicine. I'm only gonna put the clubs I actively participated in on my CV.
I never joined SOMA because I knew I'd never use it or attend any meetings. It's just not worth the money IMO. Don't join for the free book. What is the book? Didn't it used to be a Netter's Atlas? It would be cheaper to buy the book on your own.