Which standard zoom lens would you recommend?

[khaymond]

I am in the market for a new lens and debating between these four lenses:

Sigma 17 - 50mm f/2.8 EX DC OS HSM Auto Focus Wide Angle Zoom Lens
Tamron SP AF 17-50mm f/2.8 XR DI-II VC (Vibration Compensation) LD Aspherical (IF) Zoom Lens
Nikon 24-85mm f/2.8-4 IF AF-D Nikkor Wide - Telephoto Zoom Lens with Hood
Sigma - 24-70mm f/2.8 IF EX DG HSM Autofocus Lens for Nikon AF

My main purpose for this lens would be family portraits, children, couples...

I already own a 50mm 1.4, but I need to be able to zoom out.

Any suggestions?

Thanks,
Kristie

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[tomwilson]

I have personally put up the Sigma lenses against the Tamron and Canon L lenses. Of course the Canon L was sharpest but the Tamron was very close. Sigma was no where near as sharp.