This is half just to get this subforum off to a good start, half because I am actually passively (2-5 years) looking at these.

If the ultimate in image quality is the goal, but smaller size and lower price are preferable, which super telephoto lens would you choose between all available systems, and why? Equivalent focal lengths around 600mm or more, with teleconverters attached if you want.

For example, I'm pretty impressed by the Canon 300mm f/4. Looks astounding, the size is not bad, and the price is low enough I cold get it and a DSLR camera for less than the price of a similar Olympus prime alone. Even better, it has great new teleconverters available.

Alternatively the Olympus 90-250mm f/2.8. Great image quality, though not quite up there with top-of-the-line primes, it is far better than any mortal zoom lens can dream of becoming. This is made up for by the fact that it zooms, and Olympus has a superior 1.4x teleconverter that effectively makes it a 126-350mm f/4 zoom lens with little discernible loss of quality. However, it is huge and expensive.

What would you choose?