As a counterweight (you can find this test and several others via Google):http://www.coldsiberia.org/phototest/EF_300_IS_test.html"The differences between the EF 300mm 4.0 non-IS and the EF 300mm 2.8 IS are rather negligible, whereas the differences between the IS and non-IS versions of the 300mm 4.0 are very significant. This disparity in optical performance between the two latter telephoto lenses is readily apparent both without and with the use of the Canon 1.4x Extender."
One person's "photographically insignificant" is another's "very significant". And that is the only significant difference I can find at a glance in the two tests. Even the one Yakim quotes clearly shows that the f/4 IS is inferior, but whether it matters or not is up to the photographer and his/her audience, if any.
The weight considerations and IS/non-IS are of course of importance to whether the lens will be used, or whether you'll get the shot. But the non-IS versions (f/4 or f/2.
are not manufactured anymore.