The FOV crop is 1.6 on the 400D, but close enough

Unfortunately I live in Sweden, so no cheap lens rentals around here.
Now there has been quite a few that has suggested the 24-105 f/4L. What's so great about it?

Why is it better than a Sigma/Tamron 24-70 f/2.8?
Just want to know if it's THAT much better, since I can get 3 Sigma/Tamron 24-70 for the price of 1 Canon 24-105.