I prefer the OEM version. I enjoy being my own technical suppport. I can find answers to any problems I have searching online. The only reason I would pay for full version was if I felt I needed tech support from MS. I have never had a problem replacing a defective motherboard and getting my installation activated. MS has been fair to me.
The difference between the two is the Full version allows you to move the OS from one box to another (only one activated at a time, is the rule).
The OEM is supposed to be used only on the individual computer it was first installed on. You're not supposed to move it. The exception is when repairs dictate swapping a motherboard, etc, in which case you may need to call Microsoft for a new key if the licence protection activates. Takes only a few minutes.
OEM all the way. By the time you are ready to upgrade, there will be new version of Windows. I learned a long time ago to quit wasting money on upgrade or full retail versions. Especially if you are a build your own guy!
The previous posters are entirely correct. But, let me add another option.
If you get an upgrade version, it will cost about the same as OEM. But, it is considered a retail version which gives you portability and support. You also get both a 32 bit DVD and a 64 bit DVD. The drawback is that it must be installed under control of another OS.
There is a legitimate two step instalation process to install an upgrade version.
You install windows-7 from the cd, but do not initially enter the product code or activate.
After it installs, you have a fully functional OS for 30 days.
Step 2 is to insert the dvd again, while running Windows and then do an upgrade.
This time, enter your product code, and activate.
After activation. you may delete the initial version which is named windows.old.
How do I know this? Well, a while back I called microsoft support to ask how I could upgrade my 32 bit vista to 64 bit vista since the 64 bit installer would not run on a 32 bit OS. They walked me through the procedure and it has worked well. The same procedure works with windows-7 as well for the same reason.