Two considerations when it comes to 3rd-party lenses:
1) There are always good and bad copies when it comes to particular lens models. But you are getting more uniformity when you stick with Canon, Nikon, etc. You might get a great Sigma, Tokina, Tamron copy that is good as the major manufacturer, but your chance of getting stuck with a dog are much greater.
2) Resale. Canon, Nikon lenses hold their value much better if you ever decide to sell later. With 3rd party lenses, you WILL take a loss, regardless of how well you have taken care of it.