I've seen two really EXHAUSTIVE tests of the new Nikon-made aspherical element, version "III" teleconverters...very carefully-done tests with good subject matter, done under good atmospheric conditions, with lots of sample images to choose from to represent each lens setting and shooting parameters, so as to avoid bad samples with focus errors, and the "new" Nikon aspherical-element converters when paired with the "NEW" 300/2.8 VR,the 200/2 VR,and the new 70-200 Mark II were really, in some cases, MUCH better than those lenses with the older converters Nikon has been making for 10+ years now. One pro shooter's comparisons also added the 400/2.8 VR and the 200-400 VR Nikkors to the mix. The thing is this: the very-newest TC units are pretty well-matched for the very-newest lenses, as a whole. Nikon's TC's have always been designed and intended for single focal length telephoto prime lenses, with "under 200 mm" and "300mm or longer" lenses each having had their own optical designs for almost 20 years with the TC-200 and TC-201 and TC-300 and TC-301 converters for the manual focus lenses made from 1976 to the 2000's; the AF converters were designed SPECIFICALLY for AF-S telephotos, and will NOT, I repeat will NOT mount to many zooms and primes due to where the front elements of the TC unit are going to bash into the rear lens elements on many lenses.
I'm not as familiar with Canon TC units, but one thing I do know about the Sigma-brand TC's is that they have a PRO-series that has a rather deeply recessed front element, so that allows it to mount on many,many more lenses than TC's that have their front elements rather far forward. The Sigma PRO-series 1.4x converter I have seen used by many people on non-Sigma lenses, with good results in some cases. What I have found, and others have too, is that "Occasionally" there is a lens/converter pairing that is quite good to very good; there are also some that are very poor;now that we have 16-18-24 MP cameras, to me the idea of a 1.4 x 200-280mm f/4 zoom is not really all "that" desirable or versatile...when we had 2.7,4.2,and 6- and 8-MP cameras, the optical boost from a TC unit was valuable...nowadays...not so much, unless it's being used on a killer-sharp 200,300,or 400mm prime lens in order to make a longer lens with ample quality to spare. One suggestion: Sigma 100-300mm f/4 HSM as a very versatile fast-focusing sports lens, with their 1.4x added for longer-reach needs in good light. Canon's 100-400 trombone is like a dinosaur leg in actual handling....it's a dog...the Sigma 100-300 f/4 is like a dream in comparison...especially for a woman, I would think...if you have not actually shot the Canon 100-400, it's hard to imagine what a slug it is, in several ways...