The 20mm difference
When shooting with the two lenses, the most immediate difference would be the difference in focal lengths. However, considering that the actual difference amounts to about 30mm (multiplied by crop factor), it is not insurmountable by taking a few steps. However, the issue of perspective previously discussed would remain. Does that mean that the 17-70mm should be the obvious choice? The answer is of course not so simple as everything in photography is about trade-offs. With a longer lens, you will naturally lose out on clarify and so the image quality would be inferior. How much worse would vary from case by case but in this comparison, I would say the difference is not as pronounced as I would expect but you can expect the Tamron to shoot better within its range, especially as it would be stopped down slightly to give like for like comparison against the Sigma. Keeping in mind that lenses are usually at their best when stopped down 1 to 2 stops from the maximum aperture, it's no surprise that the Tamron is a touch better. Mind you, it's not enough of a difference to write off the Sigma 17-70mm. Keep in mind though that the Tamron can be likened to the Canon 24-70mm f/2.8 in comparison to the Canon 24-105mm f/4
For those who demand higher quality, the 24-70mm is the obvious choice while those who demand flexibility will go for the 24-105mm. It's really down to what you need for your shooting.