i have the old Tamron 70 - 300mm i had it come as part of a bundle with a kit lens back in 2009
now ive read the reviews of the new 70 - 300mm but you have to ask you self this your looking at Canon L lens the reviews on the 70 - 200mm f4 say that if you don't need the f2.8 version then the f4 version is just as good if not sharper then the f2.8 version by canon
but again it comes down to price because it as a L on it your paying for it back in january this year i shot a friend doing a tandem parachute jump now the old tamron lens cost brand new £99 you would not have thought it when seeing the photos it only cost that price just ment i had to work that much harder to get the shots that all now if that would have been the Canon 70 - 200mm L lens ok i would have lost 100mm but would not have had to work as hard as i did to get the same kind of shot
as ngas69 as said the canon L lens is the better lens but as wave as all so put what do you want to use it for
hope this is some help
all the best
While the Tamron 70-300mm gives you extra reach and performed very well in our Telephoto Zoom group test in the Jan issue, scoring 89%, our choice would have to be the Canon EF 70-200mm f/4 L USM.
While it costs another £100 more, its a sound investment. It may not have Image Stabilisation (IS), but the constant f/4 maximum aperture offers plenty of flexibility and has a great set of optics that won't disappoint. It's a cracking lens.
Before you make the plunge, read some reviews and take suggestions from the forum. Don't aim too low when choosing a camera. I've never heard anyone complaining that the camera they bought is too good.
Don't rule out used and refurbished cameras from dealers and manufacturers.