There is a major difference in how these two systems work.
Onstar is a concierge service--when you press the little blue button, you initiate a voice call to a data center where someone will offer to help you. Likewise, when something goes wrong, they can call you and ask if you are OK. Onstar has a subscription fee.
Sync is a speech recognition system. You don't talk to a human, you talk to a Microsoft box which integrates some functions of the car with some functions of your cell phone. Sync does not have a fee.
Both systems offer 911 functionality, Onstar through a concierge and Sync through an automated dial-out using your phone.Onstar advantages: the phone is built into the car, so it is always on, you can talk to a real person, Onstar can do things like unlock your doors or stop your car remotely if it is stolen.Onstar disadvantages: it has a subscription fee.Sync advantages: it is free. It has a nice interface for iPod control, will read text messages to you, get news and weather updates, etc. Sync will have apps that users can download and run, and there will be more of them over time.Sync disadvantages: If you don't have your phone with you or your phone is damaged, Sync can't help you as much. Sync can't do things like immobilize your car or unlock your doors. Some phones or devices aren't fully supported, depending on how they implement bluetooth and other details.