The Canon EF 75-300mm USM III lens costs 179$ on Amazon whereas the Canon EF 70-300mm f/4-5.6 IS USM lens costs 500$. Now the differences I see are:

• The 70-300mm has IS
• The 70-300mm has 8 diaphragm blades whereas the 75-300mm has 7
• The 70-300mm has 10 groups/15 elements whereas the 75-300mm has 9 groups/13 elements

Other than that I don't see any differences.

Does the 70-300mm have better image quality? Or is it just the IS that is worth the extra 300$?