Differences between various official Microsoft Windows .iso files

I want to make a bootable USB thumb drive to install Windows on my computer. I don't want to use my old DVD to create an image and then upgrade to SP1, so I decided to download an untouched .iso file that already has SP1 in it. My question is this, is there a difference between these two files files?

en_windows_7_home_premium_with_sp1_x64_dvd_620851.iso

vs

en_windows_7_home_premium_with_sp1_x64_dvd_u_676549.iso

What does dvd_u mean, is it somehow different from plain dvd? Also will these work with my retail Home Premium key or do these work only for thoe with TechNet/MSDN accounts?

Answer

the u version is more recent according to this site
check this for details