What are the differences between these two Apple TV models?
Are you in Canada by any chance? The MD199C/A code seems to be for the 3rd generation Apple TV in Canada, whereas the -LL/A code is the part number in the US. I'm not aware of any differences between the two (other than the limitations on Canadian iTunes, such as more limited iCloud TV/Movie accessibility).

You can verify these numbers by adding the Apple TV to your cart in the US or Canadian online Apple Stores.