Canon has no more than two L lenses sharing any same focal length or focal length range with one exception – they currently offer 4 different versions of the EF 70-200mm L lens. More choices are great, but more choices of course lead to more difficult decisions. While some may desire to have all 4 of these lenses in their kits, most do not need or want to afford all of them and one or two need to be selected. However, there is enough difference between the f/2.8 and f/4 models to make having two of these lenses in a kit make sense. The No-Brainer Choice
The bottom line is that, if size, weight and price are not issues for you, the Canon EF 70-200mm f/2.8L IS II USM Lens
is definitely the lens to get. This lens offers the best of everything and it is the most versatile among the 4 options. The Rest of the Options
The decision becomes harder if the f/2.8L IS II lens is not affordable or if size/weight concessions are necessary. The 4 lens models are separated, in specs at least, by having or not having IS and by having an f/2.8 or f/4 max aperture. Fortunately, the remaining three options do not sacrifice performance as they all reliably deliver great image quality.
If size and weight are concerns, the f/4 models are the direction to go. At roughly half the weight of the f/2.8 models, your arms, shoulders, etc. are going to clearly know the difference after hours of carry and use. At roughly half of the cost of the respective f/2.8 model, your wallet will understand the weight difference as well.
Image stabilization adds to the cost, but it also adds greatly to the value of the lens, adding a great amount of versatility. If handholding in low light with still subjects, the f/4L IS model is the right choice between the f/4 models and possibly the better choice over the f/2.8 non-IS. This lens features weather sealing like it's f/2.8 counterpart (filter required) and delivers better image quality than the older, non-IS model
and it is arguably better than the f/2.8 non-IS
The f/4L non-IS has the most attractive price tag and has been the introduction to Canon's L-series lenses for a huge number of photographers. If your budget is a primary limiting factor, the 70-200mm f/4L USM is a very capable lens with inherent benefits far exceeding its cost. It’s small, easy to pack, solidly built and a great introduction into Canon’s highest-tier lenses. However, caution should be exercised as weather sealing does not come with the budget price tag.
If stopping action in low light is important, the f/2.8 non-IS lens likely has your name on it at this stage of the selection process. The 1-stop wider max aperture can stop action in 1/2 as much light as the f/4 options at the same ISO setting. The wider aperture can also create a stronger background blur. Note that the f/2.8 non-IS version is only partially weather sealed.
As mentioned, the difference in features between the f/4 and f/2.8 lenses is significant and I currently have both the f/4L IS and the f/2.8L IS II in my kit. Both see frequent use.