Canon 70-300 vs 70-200L Comparison

Both are very common telephoto lenses. Now which one to buy? My thoughts..

Canon EF 70-300mm f/4-5.6 IS USM
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D Adv
Not compatible with the Canon 1.4X and 2X extenders
Rotating Front element -If planning to use a polarizer you wouldn't want this.

Adv
IS - The IS is 3rd generation and will achieve 3 stop adv - Good for hand held shots. Esp at the telephoto end.
300mm at telephoto end.

Canon EF 70-200mm f/4.0 L USM Lens
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Adv
Fixed size - Size does not change while zooming - I personally like this feature.
Non rotating front element.
L quality glass.
Fixed wide aperture (4) - Good if you use zoom a lot. Other wise after zooming, you might need to change the settings (recompose).
Compatible with Canon 1.4X and 2X converters (With 2X no autofocus unless you want to do the 'tape tricks')

Compared to 70-300mm f/4-5.6 IS USM,
Auto focus is faster.
Sharper.

Dis Adv
No IS. (IS version is available - for more $$ :-))
$$
Only till 200mm at telephoto side. Have to use extenders.

Neutral
Weight is same for both the lenses.
If you use a 1.4X extender, the L lens will become 98-280 f/5.6 which is almost same as the other lens.

Difference between X notation and A-Bmm notation

Actually 3X, 4X etc is a relative term.
It is used mostly in point & shoot cameras which is meant for 'all-purpose'.
The zoom for the point and shoot camera is considered as max telephoto/max wide angle.

Most of the p&s camera max wide angle starts at 28mm. So a 10X p&s means 28-280mm.(280/28 = 10x).

Now for an SLR lenses, term nX is not used. Coz not every lenses are general purpose lenses.
One example..
Assume there is a lens 10-40mm is 4X
similarly a 100-400mm is also 4X.
These lenses will be used for different purposes. First one is a super wide angle lens and will be used mostly for landscape photography. Second one is a super telephoto lens and will be used mostly for bird/insect photography.

In a nutshell the X notation is a relative term where as A-Bmm is an absolute term.