Ultimate gives you the following which Pro doesn't:
Help protect data on your PC and portable storage devices against loss or theft with BitLocker.
Work in the language of your choice and switch between any of 35 languages.
If those extra features are worth the extra $20 (approx) then Ultimate is the right choice. If you don't consider those features to be worth paying that much for then Pro is the right choice.
So for many people it will be a matter of whether it is worth paying the extra $20 for Bitlocker so as to encrypt your files