Over on 24 hr campfire in the photography section is a Tamron Rep. No need to ask him about len's, he will recommend a Tamron. But dig through and find some of his photo's, terrific. I think how much you plan on shooting with a len's should help figure out what one you get. Then your own skill level. Take someone like me and I take photo's I like to look at and tech things in the photo I seldom see. A nice photo to me is a nice photo. I can't see the things in a photo well enough to get critical with a photo. A high dollar lense would probably be wasted on me. So would it on you? I have a 55-300 Nikon lens, not high dollar at all and I have a 70-300 Sigma, again even less of a high dollar lense.Both take photo's that satisfy me. What I do have a pretty good grasp on is the difference between $350 and $1000! Most pro's say go with the camera manufacturer's len's and usually the more they are, the better they like them. Seem's that most really good photographer's chase that elusive perfect photo but most everything I look at of their's is perfect. Even if you do it for money, how perfect does the photo need to be? The things that one guy doesn't like about a photo, a really good photographer tears apart and 98% of the rest of the people think it's great. Most people thing a good photo is only made by a good camera; the old "You must have a camera" ploy we all like to hear.
You can invest a lot of money into lense's that take better photo's than you do or you can spend a lot less on a like lense from an after market maker that takes photo's most people think are great. With the difference you can buy another good lense.
I have sold rights to many photos over the years. If you sell photos, they need to absolutely perfect. Stock photography is inspected thoroughly before it is accepted into a stock library. It is zoomed up to 100% and if there is noise, motion blur, purple fringing, lens blur -- it gets rejected. A lot of us have worked hard to improve our photography throughout our adult lives, from film to the new digital format. It takes a really good lens to take really good shots.
It would behoove any ambitious photographer -- who enjoys learning, and seeing himself grow as an artist -- to buy the finest lenses he can afford.