Both lenses have image stabilization. Both lenses use ultrasonic focus drive that should be reasonably quiet, accurate and fast... although the Tamron USD will not be quite as fast as the Canon USM.

Tamron lenses... especially their "SP" pro quality... are very good and I think usually have a 6 year warranty, if there are any quality control "issues".

Both lenses purportedly suffer from the same thing... they aren't quite as sharp at the middle focal lengths.

But, in this case I'd likely get the Canon instead... because it's smaller, lighter and offers very, very good image quality.

It also offers exceptional close focusing capabilities, 0.70X magnification is close to what a true macro lens can do and two to three times the magnification possible with most lenses of this type. It needs to be noted, though, that at the highest possible magnification the front of the lens is very close to a subject and likely to cast a shadow over it. Still, it could be a very handy feature for a wedding photographer, for example, who needs to take some close-up shots of rings, bouquets, centerpieces and cake details at an event. A zoom with this capability might make it unnecessary to buy and carry a true macro lens, too. (For comparison, the Tamron's best is 0.20X, which is similar to the Canon 24-70/2.8L II).

When you look at image quality performance, keep in mind that the 24-70/2.8L II, to which both the lenses are often compared, is called the "sharpest mid-range zoom ever" by a lot of reviewers. The 24-70/4L appears to have even better corrected distortion at the wide end, making it perhaps one of the very best of all lenses of this type in that respect.

The f/4 aperture of the Canon 24-70mm allows it to be smaller and lighter.... but of course you're giving up a stop of light. On the other hand, this (and the Tamron) are among the few lenses of this type that have image stabilization, which can partly make up for the slower aperture in some situations. The Canon 24-70mm uses a hybrid form of IS that's rated for 3 to 4 stops of assistance. It's hybrid in that it automatically detects panning and switches itself to correcting movement only on the non-panning axis, to better allow pan-blur effects.

An f/4 aperture also may not be an issue, depending upon what other lenses you have in your kit. If you have one or two or more relatively fast primes in some of these focal lengths (28/1.8, 35/2, 50/1.4, etc.), you might not need an f2.8 mid-range zoom. Larger apertures aren't just about low-light shooting.... but also about the ability to render shallow depth of field effects. However, that's not a significant factor at short focal lengths around 24mm and more dramatic at telephoto focal lengths longer than 70mm.

It's also kinda nice that the Canon sells for about 2/3 the price of the Tamron ($900 versus $1200.... and nearly half the price of the Canon 24-70/2.8L II).

You should compare the specific lenses at The-Digital-Picture.com, to see for yourself. There you can put sample test shots side-by-side.... as well as distortion, MTF, general specifications, flare and more.

Note: The relatively new Tokina AT-X 24-70/2.8 might be worth a look, too. It is $850, slightly less expensive than the Canon, said to be quite sharp and extremely well built... heavier even than the Canon 24-70/2.8 II! However, the Tokina doesn't have ultrasonic focus drive so should be expected to be a noticeably slower than either the Tamron or either Canon. It has the usual Tokina "focus clutch" mechanism, where you slide the focusing ring forward or backward to shift it in and out of auto focus.... it doesn't allow for manual override of AF without first turning off the AF (in fact, in AF setting turning the focus ring doesn't do anything... unlike Tamron USD and Canon USM where you can manually tweak the lens any time, whether set to AF or not).