The Canon 70-300mm L is not compatible with the Canon extenders, but is with the kenco.
TBH the IQ difference wont be day and night between the two in good light and low ISO.
But what will be very visible is the noise difference at higher ISO. The 40D is only just usable at 1600, 3200 forget it. So with the stops you will loose you will need to increase the ISO to compensate there the 5DMKIII will have an advantage.
TBH if you want more reach the best alternatives are the 400 F5.6 or the 100-400mm IS in the sensible bracket.
But you will get 480mm out of the 40D without the teleconverter. 672mm with a 1.4 extender.