Zoom lens for Canon 300D - Tamron/Canon

Hi,

Looking for a zoom lens with atleast 300mm at the zoom end for my Canon
300D. Initially, I had made up my mind about the Tamron 70-300mm LD but
then noticed that the Canon 75-300mm has USM. Quickly went thru reviews
and realised that the Canon 100-300mm has true USM (ring-USM vs
micro-USM) and is optically rated better than the Canon 75-300mm. Some
even compared it to its "L" series sibling, the 100-400mm L.

Anyways, a few more reviews down the line, some opined that the Tamron
28-300mm XR is sharper at the 300mm end than the Canon 100-300mm. The
photodo tests rate the Canon 100-300mm USM at 2.4 while the Tamron
28-300mm is rated at 2.6.

Then there are a few old Canon 100-300mm f/5.6 lenses floating around
at eBay.

What I liked about the Canon 100-300mm f/4.5-5.6 is that it has USM so
AF should be faster, manual focus over-ride in AF mode and rear
focussing element.

I would like to hear comments on:
1. Optical quality comparison of the three
2. Build quality
3. Suitability for 300D sensor, that is, how bad does a lens have to be
before the 300D sensor can start telling the difference?

I am open to used lenses and my upper spending limit is US$200.
Thanks,

Siddhartha