Either or, as far as I know. Remember that as a system builder you are supposed to install the OS onto the PC before you sell it, thus you only get either the 32 or 64bit edition.
You only get the version that you order, either 32 bit or 64 bit. As a systems builder you are expected to know which version you will be installing.
If you are expecting to change later, you will be better off to buy the retail version.