Mid range zoom
David, why not the new Tamron 24-70/2.8 VC...? I´m sure you will appreciate both the image stabilizer and the f2.8 for playing with the depth of field (which is a strong creative aspect in certain situations the 4.5-5.6 zoom cannot really give you).
p.s. Yes, the Sigma 24-70 is not an example of well-done lens... It is really below average in optical quality - don´t spend on it, sooner or later you would regret it. Based on my experience, if a more expensive thing brings a real quality, the initial high (and rather painful for sure) investment becomes soooo relative in the long-term satisfied use...