I used this Tamron lens with my old film SLR & didn't like it. I tried the Canon 75-300 USM
III once (photojournalism exercises for a seminar, shooting slides). From this impression & the photos in the database shot with this lens I'd say the Canon is clearly ahead of the Tamron.
My Tamron showed both visible vignetting & chromatic aberrations. There was also some distortion at the edges. This might not be a serious problem though as the 300D crops the edges (1.6x crop factor). Another problem was softness: I couldn't get any tack-sharp photo at 300mm. The quality of colours also deterioted. Most photos looked pale. Autofocus speed was OK
under good light conditions, but don't expect miracles. Overall, I can't really recommend this lens for aviation photography. It might be good for other kinds of photography, though.
Invest some more money & go for the Canon. As I said earlier, prices on the Canon website are suggested retail prices. You can easily find one for €250.
Even a broken clock is right twice a day.