It doesn't seem like it will necessarily be a disadvantage. I joined Alpha Omega during my sophomore year and didn't have to do any silly initiation rituals like swallowing live goldfish. All it took is writing a check for $100 and I was in. I think it will be good for after graduation when I try to move somewhere new and I don't know anyone in the dental community. At least I could send an e-mail to the members in the area and let them know about a newcomer to the area and hope someone takes me under their wing. Also, they offer 2 week internships during the summer where all you do is meet member dentists in the area of your choice. A student at my school did it and said it was a really good experience, so I may consider that next summer. As to which fraternity you join, I think it depends on the personalities of the frats at your school. Between DSD and AO, I just thought I interacted better with the AO people at my school. I've also met a lot of upperclassmen I wouldn't normally see and got to know them better through AO.
Is it worth it? If you have the cash, then sure, why not? It doesn't hurt to be a member.