GroupMe - the best of all the services.
-Works on iPhone, iPad, Android, and web (simultaneously with one account) - this is probably the biggest differentiator
-Can change the group name for everyone (helps with target spamming)
-Can kick people from the group
Cons:
Limit of (maybe?) 50 people until you email support (pro: they don't take 2+ weeks to respond with an automated message hoping your issue is fixed)