Websize photos will hide things. Canons USM AF motors will certainly be a lot more faster and quieter than a Tamron. The Canon will not sound like you have a captured bumble bee in the lens.
If Weddings were my bread and butter I would use the Canon for speed and accuracy. The Tammy is no doubt an incredible bang for the buck but I would have more confidence in the Canon lens for crucial photos.
I chose a Sigma 18-50 many years ago because I liked the build more than the Tammy (this current setup is my wifes XTI/lens combo). If I decided to go for a 1.6 crop again I would no doubt purchase a Canon 17-55IS. MY motto is cry once $$$ so theres no buyers remorse later.
Very true. My friend followed me and got himself a f/2.8 lens. He got the Tammy and I got the Canon instead. Both look remarkeable in terms of IQ. I say the IS helps me a little more in low light indoor shots.
It is true the 17-50 is loud as hell in AF. When we were shooting in more quiet areas, it was so freaking loud while my 17-55 IS was dead silent
The 17-50 sounds kinda like the Tokina 11-16 in terms of AF noise, except it's about 10x louder. It's also louder than the 18-55mm kit lens. And yes, it does sound like a high pitched buzzing bee.