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$. Now the differences I see are:
Other than that I don't see any differences.
Does the 70-300mm have better image quality? Or is it just the IS that is worth the extra 300$?
The 75-300 is generally considered to be one of Canon's poorest lenses; the IQ from the 70-300 is significantly superior. See for example The Digital Picture's reviews of the 75-300 ("Image quality from the Canon EF 75-300mm f/4-5.6 III USM Lens is also mediocre.") versus that of the 70-300.
I had the 75-300 (Non-USM) for a while, which I actually got on ebay new for $99 US. The only positive feelings I have about that lens is that it covers the 75-300 mm range.
It was a pretty poor build quality, exhibited quite a bit of chromatic aberration, was slow to focus, and was soft wide open. The softness with a wide aperture combined with the focal length, made it very difficult to hand hold in anything but daylight.
The USM version should remedy the slow focusing speed, but the rest of the issues will remain.
The 70-300, which has both IS and USM, should focus relatively fast, and should be usable in a wider range of lighting without a tripod. I haven't actually used this lens, so I can't say for sure, but everything I read leads me to believe that the image quality is significantly better as well.
