To my way of thinking buying a function generator is like buying any other piece of test, measurement or diagnostic equipment. You generally select it based on your needs and budget. With for example a function generator do I want or need sweep functions? How much amplitude do I need. Sometimes the basic features are fine but sometimes we need additional features.
Also, depending on your location frequently a good deal can be had on used test equipment. I got the old Wavetek Model 145 pictured in the attached for $5 (USD) at a yard sale. That thing has to be 25 or 30 years old and works just fine for many applications.
Ron