OEM supplied via such as ebuyer will be much cheaper, without Microsoft support and once registered tied to the machine it's installed to. None of which seemed a problem to me when I did my homebuilt, so I went OEM.
If you ever developed a conflict,(which I have) in your OS and would like to reformat you OS, or repair some fault, it is straight forward with a full retail version. With an OEM it is a nightmare unless you know how to make an Image and then recopy all your programs back to your reinstalled OEM OS from reinstall CD's losing all your other data that you have not backed up. The extra cost is worth that peace of mind, if it is your own computer and you do not like hassles and you are not an IT Whizz kid.
I agree with Belatucadrus, I have built abot 10 computers for friends and relatives and have put an OEM version on all of them. The full retail version can be transfered to another machine provided you have wiped it of the previous computer, the OEM version cannot as was said above it is tied to one computer.