Professional is a strip down version for business type applications (not all of the extra junk) and Ulimate has everything (the extras!!). A lot of people like the Pro, since it doesn't have all the extra junk that takes up hard drive space / resources.
Looking into buying Windows 7 for my new computer. What is the difference between Ultimate and Professional Version?
Everyone overlooks the most important difference, support.
Pro gets extended support. That means that when mainstream support ends in 2015, it continues to get security updates through 2020.
Ultimate does NOT get extended support, so security updates end in 2015. That is the biggest difference.
If you think you will still be using this OS in 2016, Professional makes a lot more sense. You might need some of the oddball stuff included in Ultimate, but it is extremely unlikely. So I would say 7 Professional is the best choice.