I use my Tamron 17-50 quite a lot on my 70D and it works fine through the viewfinder. Live view is a whole other story. Something about the live view AF causes this lens to hunt in little chunks at a time until it eventually focuses. I'm talking minutes. The lens also works OK in video until your focus target changes significantly and then you may as well just stop recording.

The story is almost the same for the Sigma 18-125. The AF motor on that lens is a little bit better than the Tamron and it has a similar hunting issue in live view. It seems like the logic on 3rd party lenses is for the AF to front focus, step in until back focused just a little, and then step back to lock. The Sigma is about a 5 second process.

The speed of the AF motor in the lens will likely be the difference maker in how well each lens works in live view. The Canon lenses are much better, but the crappy motor in the 50mm 1.8 II struggles to keep up in Live View as well.