Canon EF 70-200mm f/2.8L IS II - or - Canon EF 100-400mm f/4.5-5.6L IS II

Pietro.

First off, it sounds like a question of which you First.

A little history on me. My first "L" lens was the 100-400 L IS version 1. In 2008, I actually acquired this as a barter from my old business not related to Photography. One of the advantages of both parties being self employed. Once you have an L lens you are hooked. Having 100-400 range I was able to capture images that were not possible. I'm sold. Soon after, the 24-105 made its way into the bag.

Then I stated to get the 70-200 IS bug. In the summer of 2009, the choices were the f/2.8 Version 1 and the newer f/4 L IS that was outperforming the f/2.8 IS in terms of image quality and improved IS. Plus the f/4 L IS was half the weight and almost half the price. Plus there were rumors of an f/2.8 on the horizon. So I bought the f/4 L IS. Great results were captured in this smaller lighter lens. I knew I could always sell or trade in the f/4 L IS toards a new 70-200 f/2.8 L IS Mark II down the road. Another 18 months Down the Road it was time to invest in the 70-200 f/2.8 L IS Mark II. Weight not being a concern at the time as I was able to get that addition stop of light with slightly improved optics. So the f/4 sat at home as I decided when I was going to unload it. It sat and sat in the corner waiting to see the light of day.

One day when the shoulder was acting up, I  decided I was going to use the trusty f/4 L IS as I was going to be walking all day and wanted to lighten the load. Since my old business was supplying the Physical Therapy industry, why put my body through the additional stress when I had a solution. Sure glad I had the f/4 on reserve that day as shooting f/2.8 was not necessary.

To this day 5 years later, I still use both the f/4 L IS and the f/2.8 Mark II. Obviously the choice to keep both has paid off for me personally.

My recommendation to your question is to go after the new 100-400 Version II as it allows you more areas of Photography than the same range with one additional stop of light.

I am really looking to upgrade 100-400 to the new Mark II. Everything I am hearing is that the upgrade is well worth it.