24-120 f4 vs 28-70?
I owned the 28~70 when using my D200 and expected to use it alot on my D700. When the 24~120 f4 was launched I bought that thinking it's range would be good as a walkabout lens, plus I wanted it for events. The first event I used it at was my Dad's wedding!
Long story short, I sold the 28~70. I decided I didn't have a use for it any more. It's a big, heavy lens with limited range and not much reach on FX (principally, not as long as I'd like for portrait). The IQ of the 24~124 is excellent, sharp, nice bokeh, good wide open. It suffers from some distortion but it is not usually problematic and in any case it is easily fixed with modern software.
Ok, I have given up a stop but I don't regard f2.8 as fast anyway. If I want fast I go to my primes. I've got a 28mm f1.4, 50mm f1.8G, Sigma 85mm f1.4, Nikon 135mm f2 DC.
I was a happy user of the 28~70 for a few years but I have not missed it.