Tirandia – if you can afford it, do yourself a favor and get the 2.8.
I have had both versions of the f/4 and both versions of the 2.8, I know currently have the 2.8 II with the IS.
The advantages of the f/4 are the weight savings, portability, and image quality. If you do any kind of action photography, the IS will come in handy more than you’ll know. The good thing is that it is very fast and very responsive, but will drain the camera battery down a bit whenIS is used.
The 2.8 is a far better lens, but it does come at a substantial cost in both $$ and weight. Now in comparison, the cheaper of the bunch is the non-IS f/4 but you also save because it has the fewest elements of the 70-200mm group, only 16. The IS f/4 version has 4 additional elements, 20, so your not only paying for the IS but some additional glass.
The f/2.8 versions also differ slightly between the IS and non-IS versions. The non-IS f/2.8 has 18 elements where the IS version has 23 but also has a fluorite front element.
In my opinion the Mark II version of the f/2.8 with IS is the best of the bunch, but it comes at that added price. I understand, if your on a budget, the f/4 is fine, I had one and the images are excellent from it. The f/2.8 will allow you that extra stop of light which will help in those lower light situations, just like CC stated.
I loved my f/4, but wanted something better, so I got the 2.8 and kept the f/4 for a backup and those shots where the 2.8 was just too big and heavy to use.
Good Luck and if you get the 2.8, you won’t regret it.