Low-Mid range 'zoom' lens f/2.8?

[MadTester]

Hi All,

I'll probably get alot of stick for asking this question but I have done much research on the web but I still have doubts over which lens to get (apart from leaning towards Tamron solely for the price).......

Sigma 24-70mm f/2.8 (nikon fit) or the " equivalent" Tamron 24-75mm... I like outdoor photography and have the sigma 120-400mm (monster) literally permanently attached to one of my nikons....

I would however like something a little more portable and more responsive in low light.... anyone here use either or both these lenses and what are your experiences with them?

Any advice would be great... thanks in advance