Mid range zoom

Ideally would like the Nikon 24-70mm f2.8 but at £1200 its more than I can really justify as an amateur. The new Nikon 24-85 vr sits in the same price bracket as the Sigma 24-70 2.8 HSM ie £500-600 in the UK. The Sigma comes with a 3 year warranty compared with the 1 year for the Nikon. The reports suggest the Sigma on FF (D700) needs to be stopped down to f4 to obtain good IQ but thats only where the Nikon 24-85 starts. Main worry is the autofocus ability. Advice from those who use these lenses would be appreciated.