To me Microsoft seems like the clear bet. Higher pay, and smarter people. Spend your first 2-3 years gathering industry knowledge, learning from the smart people, and building yourself a nice nest egg. If you end up enjoying it, you can climb the corporate ladder. If not, you should find it much easier to interview for a new startup with Microsoft on your resume as it holds more street cred. It is also easier to return to corporate (in the event your startup fails) with prior corporate experience.
The only danger of going corporate first is that you get too comfortable and don't want to leave your paycheck. You get used to having money unlike in college when you had none. If you make $100K/yr at corporate and need to downgrade to $70K for a startup, you really feel the bump. Just be ready for that.
I speak from experience. I graduated, took the corporate job and saved up a ton of money over 2.5 years and I never regret my decision. I saved up, built some corporate connections, and recently quit. In fact, people expected me to quit (I was always hacking on side projects and keeping my knowledge up to date), and they said to me "Good luck. If things don't work out look us back up.", no bridge burned.
EDIT: I just realized a con was "hard time socially connecting at Microsoft". This is actually a red flag in my book. Can you elaborate? Why would they offer you a job if you didn't get along socially?