Sorry, but there is one other important difference. The OEM one is licensed to only be installed and run from one machine. If you get a new computer, you cannot simply get rid of your old computer and install OEM on that computer using the same license. You can, but you'll be in breach of the agreement. The retail can be installed without limitations as long as you're using only one machine. If I buy a new computer every month and throw away the old and just install the retail on my current machine, its fine.
Honestly, you'll never run into trouble with that. I've had OEM versions in multiple different machines (one machine at a time) and activation has always worked fine.