I can confirm that primes are certainly sharper than zooms. It's a given that the sacrifices that are made to a lens to allow it to zoom, reduce the ability for it to perform at its best. Primes are designed specifically for their given zoom.
I think however that you may be experiencing one of a number of possible problems
With the 75-300mm at least, try to stay away from f4.0 or more wide open. The consumer zooms are terrible performers at these apertures. I suggest f7.1 or f8.0 at least - otherwise softness will be encountered.
Movement & ISO
In some situations, using f7.1, f8.0 will force you to use lower shutter speeds unless you wish to alter the ISO setting. At the lower shutter speeds (e.g. 1/60), panning with a subject to take a single shot, you'll produce hit and miss results. Try taking a few shots by holding down the shutter release, and pan with the subject. This will reduce the movement associated with hitting the shutter release and produce a couple of good results in a batch of bad.
If you have IS
, resort to this for the static shots, otherwise bump up the ISO. At around 800 ISO you'll be getting as noisy as our point and shoot counterparts do on a daily basis at 100. If the noise is still unacceptable, try www.neatimage.com
Unlike the point & shoot, rangefinger digitals, the Canon DSLR range are designed from the ground up to produce slightly soft images. In this un-retouched form they are more suit to publishing. The 300D is a little sharper that the 10D because the parameters are shifted up a notch (sharpness 0 on 300D is like +1 on 10D). Canon's covering those amateur photographers who will insist on making prints straight from the image found on the CF without visiting Photoshop first.
If you don't edit your shots, and resort to USM
- you're missing out.
Last but not least, take a look at Pro versus Consumer lenses
Respected users.... If my replies are useful, then by all means...