My Sigma 10-20 is generally attached to the body most of the time.  It really depends on where i am going and what i'll be taking photos of / my aim for the day.  My "lite" pack (sling bag) at the moment is my 10-20, 8mm fish and my 70-200. 20-70 gap you say... i hardly take anything at that range it seems. Sometimes i swap the 8mm for my 50mm.

And i have heard similar rumblings about a 10-24mm tamron... might be good to have the extra range i suppose and it will be a "new" lens which hopefully means better than the last "generation"...

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography