I'm in the process of selecting a new lens for my camera, a Nikon D7100 that works perfectly for sports, birds, landscape, street and people photography. I'm new to the whole photography thing (merely a year give or take) but I just don't know if I should invest in a professional-grade lens or on a 3rd party lens that brings "similar" quality to that of professional lenses.
I don't want to say that money isn't a problem at all but I am well aware that I could save a ton getting the sigma one but my thinking is: if I am going to invest in good lenses, why not go for the "professional" ones if I will eventually someday end up with a flagship lens either way? -- but then again, since I don't make money off of this and it's just a hobby I can't really justify the need of buying a $2,400 lens even though I'm taking photography courses just because I like it so much.
I've read many reviews on the sigma vs the nikon and it seems to me that for what I do and the cost of the lens, I should go ahead and get the sigma since we're talking about a $1,000 difference here.
Update: I did a bit more research and found the Nikkor 70-200mm f/2.8 VR I for about the same price as the Sigma ($1,200-1,500 in near mint condition used, 8.5/10). Should I get the used Nikkor instead?
Added (1). I forgot to mention that I've considered the Tamron 70-200mm f2.8 (latest iteration) since it is sharper than the Sigma wide open at 70mm and 200mm. The only thing that bothers me is the slow focus.
Read more: Sigma 70-200 2.8, Tamron 70-200 2.8 or Nikkor 70-200 2.8 VRII?