One of my professors talks about "ilities" a lot with software/hardware decisions. This is in the order he states them as well-
- Functionality-Does it do what you need it to? This is the number one factor to consider above all else. Even the cheapest solution is not an option if it doesn't accomplish your goal.
- Usability-Will you be able to learn it quickly enough for it to be effective?
- Compatibility-Will it mesh with your other software/hardware without problems?
- Scalability-Is it easily and inexpensively upgradeable?
- Affordability-The least important factor. If it meets all the other requirements, shouldn't you adjust your budget to get it?
Just a few things to think about. Personally, I'm in University and have an MSDNAA account, so I go with whatever is free from MSDNAA and then open source software like GIMP.