Lets see the F2.8 lens first, shooting a tree in the garden at 200mm, F8, cropped image below:
Now at F4:
And finally wide open at F2.8:
Looks pretty good, very little chromatic abberation.
Now moving onto the 70-210 F4 lens, at 210mm, F8:
The colours are not as punchy as the F2.8 lens. Now lets go wide open at F4:
Significant loss of resolution.
Now lets try a harsher test: Trees with white sky behind it for chromatic abberation. Again, beginning with the F2.8, this time at 170mm, at F8:
Pretty good. Now at F5:
A bit of focus error, yet the in focus parts still holding up well. Finally wide open at F2.8:
Finally see a bit of chromatic abberation. Still acceptable as it is a very harsh test.
Now moving onto the F4 lens at 210, starting with F8:
Hmm, sort of acceptable, but it should be better than this at F8. Now at F4:
Significant chromatic abberation!
However, the F4 is £150 or even cheaper on ebay. The F2.8 is ~£300-350+. You get what you pay for. Also the F2.8 is significantly heavier, so it is quite cumbersome to carry around. The F4 is a delight to use for street shooting at F8, and convenient to carry around.