Well, usually when you're in the BIOS, your CPU isn't running at full clock and not really doing any calculating. So you should see lower temperatures in BIOS.
Temperature wise, 50c isn't anything to be worried about. If you were hitting 80 idling, that would be a different story. Underload, if you're hitting > 80, that would be bad, 70s is borderline. The software is reading the same sensor as the BIOS is, so both are correct.