If you choose oem (system builder), it is the exact same, except you can't move it to another machine. You don't get any technical support from MS . I don't know anybody who wants technical support from MS.
Full retail you can move to another machine.
https://www.amazon.co.uk/64-bit-Engl.../dp/B004Q0T0LU
Most of the pro oem come with 64 dvd installation media. It doesn't matter, the key works for 32 or 64, but oem versions are cheapo , so they only supply one dvd. You can get the installation media from here, both bit versions
Windows 7 Direct Download Links, Official Disk Images from Digital River
You want the SP1 U ( media refresh ) . they accept all valid keys, including oem (system builder).