Iam Guessing you mean extended Service contracts
If so And if your question is which one is the best from GM the GMPP Major guard with 0 deductable would be the best Doing it at the end of the warrenty is extremly smart. Keep in mind with 99.9 percent of extended contracts they dont cover everything. Meaning its not a bumper to bumper. None of the GMPP contracts cover Trim pieces.
You can compare GMPP plans to find which plan is right for you at
http://gmprotectionplan.com/coverage_comparison.cgi
As far as third party plans are concerned its my personal advice to stay away. Working for GM i have seen many horror storys when it comes to third party plans.
I hope the little bit of information i gave you helps you with your decision. If you need anymore help let me know.