Yeah, that's why I'm always weary of aftermarket parts. With something like that if you really don't feel comfortable with it, it's sometimes worth the piece of mind to buy the GM part. If that plastic one cracks, or falls apart, you're going to be loosing oil FAST! I don't know if it's worth the risk to save a few bucks. Of course, it could be fine, and never fail, but it's a risk.
Also, for cheaper GM parts than the dealer, try
www.gmpartsdirect.com,
www.gmotors.com or
www.rockauto.com .
Good luck!