I think you will have to judge for yourself if the GM Pioneer System is worth the extra money. However, I think that buying an aftermarket stereo will be very difficult. GM uses the stereo display for controlling many of the car's internal settings/preferences, so I doubt if an aftermarket stereo is an option - but you can check with Crutchfield.
As far a bugs/problems, most people's vehicles have been virtually problem-free. Sure, there are some with little/big problems, but I don't hear of many people that are unhappy with their decision. You can do some poking around the forum to gain more insight. Keep in mind, when people have problems, they visit a forum for answers, when people are happy, they don't - for the most part.
Personally, I own a 2010 Equinox LTZ, and I have had ZERO problems, and never second-guessed my decision to buy this vehicle. Well, one thing - I don't get the advertised MPG, but I blame EPA for that, and not GM. I think the EPA test does not mimick my real-world driving, but that may be true for any vehicle.
Good luck with your shopping.