Apropo dylematu..
Znalazłem coś takiego ( wybaczcie za j.ang )

Advantages of the f2.8 lenses over the f4 lenses

* f2.8 lenses allow a 1-stop faster shutter speed. When shooting action (sports, kids etc.) this halves the amount of motion blur.
* f2.8 gives smaller depth of field and greater background blur than f4 at the same focal length. This better isolates the subject for portrait work.
* f2.8 allows autofocus in lower light than f4 lens. Since all AF is done with the lens wide open, this is true no matter what shooting aperture is set t
* f2.8 lenses may autofocus faster and more positively in low light than f4 lenses.
* f2.8 allows higher focus precision. Many EOS bodies have a high precision AF sensor which operates only with lenses having an aperture of f2.8 (or faster). The high precision sensors give AF precision of within 1/3 the depth of field. Normal precision sensors give AF precision of within the depth of field. To give an example, a 200m lens focused at a distance of 10ft with a high precision AF sensor at f2.8 should give a focus accuracy of +/- 0.4cm. With an f4 lens and a normal precision sensor, focus accuracy will be +/- 1.65cm
* f2.8 permits AF when a 2x TC is added, but and f4 lens does not. This applies to most EOS bodies (Digital Rebels, EOS 20D/30D/40D, EOS 5). Pro level EOS-1 series bodies (and the EOS-3) can AF at f8 and so will give AF with an f4 lens + a 2x TC.
* The 70-200 F2.8 lenses come with a tripod collar as a standard accessory. With the f4 versions, if you want the tripod collar you can get it, but it will cost you an extra $140 or so ( Tripod Ring Mount A )

Advantages of the f4 lenses over the f2.8 lenses

* The f4 lenses are slightly smaller
* The f4 lenses are lighter
* The f4 lenses are cheaper

Advantages of IS lenses over non-IS lenses

* IS lenses can be hand held at shutter speeds maybe 3 stops slower than non-IS lenses and still get similar image sharpness. So, for example, if you need to shoot at a shutter speed of 1/320s to get sharp shots with a 200mm non-IS lens, you should be able to get equally sharp images at 1/40s with an IS lens.

Advantages of non-IS lenses over IS lenses

* Non-IS lenses are cheaper