It is plain and simple, and easy to find, the differences between OEM and Retail, and has been covered in depth on these forums and others for years.
However, the gray area is the interpretation. I am the system builder, I provide my own support, so I should be okay to use OEM. That's in debate now, as the rules are being tightened. You also can technically get an OEM license reactivated after upgrading the computer, but you'd be inviolation of the EULA, so take that as you wish.
My general thoughts are this. If you are planning to use it on the same computer for years to come, and usually buy a new OS with each new computer purchase, you'd be fine with OEM. If you upgrade often, and plan to keep using Windows 7 through the upgrades, get retail. Besides, there are other legit ways of obtaining Windows 7 retail without paying the full blown price for one single license.
As for the Microsoft support, the reason why people don't go to Microsoft is that you have to pay for each incident, where as you can usually find the same answers through forum boards or quick searching. No one, as a home customer, should ever need to contact Microsoft support for anything other than an activation issue. Corporate customers are a different boat, so my comments don't apply.