I'd be interested to hear what Sigma say about your pics. Whatever lens you end up with, don't skimp on the rest of the kit - tripod and head come immediately to mind.
In the final analysis, both the most important bit of photography equipment and potentially the weakest, will always be the photographer. I say again TECHNIQUE! It takes a while to learn and, to use a cliche, it's a journey. It really doesn't matter what lens you start out with but an 'easier' one, which in this case probably means a shorter focal length, will enable youth at least get some decent shots whilst you're learning. Might I suggest it might also encourage you to develop other ways of getting the shots you want - stalking, use of hides, etc. As I said, it really isn't my field of expertise but there must be a ton of books/websites on the subject.
I think I read somewhere that it takes 14000 hours to master something. I don't know who comes up with these statistics but it could be about right. Having the 'right' gear is never going to be be the whole answer.
My hobby, okay my other hobby, is hunting with my Lurcher. There is a guy called John Darcy, who has written some phenomenal books on the subject - I say 'written' but it's the photographs that stand out. His wildlife photography is truly incredible. I believe his main weapon of choice is a 300/2.8. What I really find incredible is how he positions himself to get the shots. Therein, I believe, lies the difference between the shots he gets of his dogs and the ones I get of mine...