I joined both groups because they have different strengths and fill different needs. Hoosier Mamas has a very full calendar and lots of activities to do. The MOB has less on the calendar, although we hope to change that, but provides age-based playgroups, a monthly social and service projects.
The nice thing is that both are inexpensive (less than the cost of just the MOMS Club before we dissolved that), $1.50/month for both. Many moms are in both, but I have met different moms at each, so it has expanded my network.
I see no reason to be snobby and only join one. Isn't the point to do things and meet other moms and kids? The fun part is trying to keep calendars straight.