The most obvious difference is that the modern quilt guilds attract quilters who are primarily interest in modern quilts. Other types of guilds attract everybody. Nothing stops you from being member of both. Go to a meeting in each and see where you fit in the best. Age can also be a factor, BUT if you are a new quilter you may really benefit from the one that has more seasoned quilters. The choice is yours. I am an equal opportunity quilter leaning more toward contemporary, but I belong to a traditional guild. Good luck!