Lenses

Good afternoon,

As the title says I am looking for a new lens for my Canon 50D, I have the Canon 17-85 and the 70-300.
The last few weeks I’ve done some research and I am thinking of the following lenses:

1. Canon EF 100-400mm F/4.5-5.6 L IS USM
2. Sigma 50-500mm F/4.5-6.3 DG APO OS HSM Ca

Both lenses are in the same price range, the Canon is a sliding lens and the Sigma is like the lenses I already have a twist.

The Sigma has 100mm more mm’s on paper, but what is really the difference in the picture, if you go from 100mm to 200mm, it is clear to see, even 200mm to 300mm, but 400 to 500?
Can the quality of the Canon lens with my camera be used from some cropping?

Now I think that probably some people here on the forum got one of these lenses, what is your experience with it?

I’ll test both lenses this weekend, so I’d like to have some knowledge of the experiences of you.

Thanks in advance,

Maikel