Hi,
I totally agree with Jonh's comments above and will add my tuppence worth.
£339 plus postage from Kerso for the 70-200 F4L which is a great/superb wee lens, and £319 plus postage for the 70 - 300 IS but it depends on what you want to shoot as it is fairly short for wildlife, birds etc so then you have to look at the 70-300 IS one, or the 300 F4 prime, the 70-300 IS isn't quite as good as the 70-200 but not far off it either, the IS is certainly handy at this time of year. Here's how it normally pans out with the 70-200L, everybody buys one as their first L zoom but very quickly realise that 200mm just isn't quite enough and end up trading it for the 70-300 IS or the 300F4 IS.
My dad has owned both zooms and currently has the 70-300 IS and likes it but he misses his 70-200 L for some reason but he's not sure what, it is certainly faster focussing and very very sharp so maybe that's it. My advice, is buy either one as you will be more than happy with either, but be prepared to trade up the L lens for more reach soon, buy it used and save some money and lose nothing when you get rid of it.
There are plenty for sale just now ( another indication of the upgradeitis bug ) on this and other Forums.
Samples:
Canon 350D with 70-200 F4L
ISO 400 - 1/500th @ F4.0 (200mm)
Canon 30D with 70-300 IS
ISO 200 - 1/200th @ F5.6 (280mm)
My pros and cons if comparing these lens' together.
Pros of 70-200 F4L
Sharp
Resolution is superb
Colour and Contrast
Better Bokeh ( F4 throughout range helps )
Its an L so built to last
Can take a TC with only a little loss in quality
Cons
only 200mm at long end
Photozone Review Ratings of 70-200 F4L
Optical Quality:4 stars
Mechanical Quality:4 stars
Price/Performance: 4 stars
They highly recommend it
Pros of the 70-300 IS
It has IS which is a huge advantage with our weather.
Cons
Slower AF
Not as Sharp
Colours and Contrast a bit muted ( when compared to the F4L )
Photozone Review Ratings of 70-300 IS
Optical Quality: 3 1/2 stars
Mechanical Quality:3 stars
Price/Performance: 5 stars
Mike.