Watch the Birdy!
For me, this is the deciding factor; under normal conditions, it is indeed virtually impossible to tell an image shot with a Sigma from one shot with a Tamron from one shot with a first-party lens. If you're going to be using the gear professionally, as in, working it hard, and not always in the best of conditions (Everyone who's had to stand for 45 minutes in the pouring rain waiting for the late bride, raise their hands please...) this is where the extra money spent will pay off. As for buying used, heck ya' it's worth it. Out of my entire stable of lenses, I think I've only bought three brand-new. Again, this is where quality pays off... good gear lasts and wears well.I believe for the most part, you could not pick out an OEM -vs- third party lens by looking at the photos they took.
what you get from OEM stuff is typically a better build quality, sometimes better optics (if your pixel peeping on a computer), and better future compatibility.