About D700, never been happier, about 24-70 F2.8 vs 28-70 F2.8, my findings:
*While 24-70 is even a bigger lens than 28-70, it produces a very nasty lens shade until 42mm while using the D700 PUFlash. With 28-70 the lens shade disappears already @ approx 30mm (both used without sun-cap).
*24-70 is sharper wide open, but 28-70 is close. Stopped down there is no difference TMHO...
*28-70 has an aperture ring which 24-70 hasn't. It's not important to me but perhaps worth mentioning.
*28-70 is TMHO (much) better built than 24-70.
*Distortion is -also TMHO- better controlled with 28-70.
*About AF speed I did a humble test with my 24-70 here: http://www.fotografie.fr/n3-080824-AFtrackingD700.htm and as you can see it works great, I did the same with my 28-70, same result...
*Having 24mm is a big advantage for the 24-70...
*About bokeh I don't see any difference.
*About vignetting, a little wide open, but no real issue and D700 vignetting correction works great. Compared to the Canon 5D with 24-105, there is no issue @ all...(24-105 vignettes terrible on 5D)
*About flare I didn't do any testing yet, both have an impressing suncap.
*You can focus much closer with the 24-70 than with the 28-70. With 28-70 you can come as close as approx 30cm from the lens front, which is OK to me.
About less expensive alternatives:
-The 24-85 F2.8-F4 is a valuable alternative and has a half macro mode, excellent about sharpness, focusing quite fast for a screw driven lens but exhibits terrible distortion, all this @ less than half of the price of the 24-70F2.8.
-I also tested the tamron 28-75 F2.8 but I was very disappointed about it, perhaps I had a bad copy...
-My favourite in this focal range maintains the 28-105 F3.5-4.5, if this lens would have AF-S, I even wouldn't consider 24-70 or 28-70 F2.8.
