Septic,
I use the 170-500 on my Canon 10D and have been generally pleased with the results. It is heavy, and can be a little bit slow, but I can’t compare with the 50-500. You’d be able to compare weight on (I think) Warehouse Express’s site.
I’ve posted a few shots on the thread “More Fairford photos”. These were all taken using that lens, and sharpened /cropped in PhotoElements. The F-16 is amazingly sharp when I printed it. I do find that a bit of clarity is lost when posting – I have no idea why!
CargoM