I've heard this argued for years.  Generally the guy arguing for 7 1/2 doesn't have a 9 and vice versa.  We've had bit of everything, 28 ft 7 1/2", 26 & 32 ft 9", 29 ft 8 3/4", and to be honest I can't argue either way.

Typically a 7 1/2 is a little better in Spring work after fall tillage. The 9 will do better if you need to cut through residue.  The soil type affects how it works as well.

The best answer I can give is talk to your neighbors and see what they are using and why. Gramps always told us "You can't be right and everyone else wrong."  I fundamentally disagree with that, but in this case he was right.  See what is working in your area.  It will be more relevant than someone halfway across the country.