I presently shoot a D3s/D2x combination. My main paid work is architecture (interiors/exteriors) and heavy machinery on locations on farms, in forests, and industrial engines for John Deere. Some studio work for brochures for same, and on-location in-working-environments portraits. AF is the mainstay of the on-location work.
I use the 14–24.2.8, 24/3.5 PC-E, 24–70/2.8, 60/2.8G micro, 105/2.8 VR micro, 80–200/2.8 AF-S, extenders, and a number of primes. I have a bunch of SB-600s, and a SB-800. I rent lights for studio work, or use an Elinchrom three-head system.
I have pro video gear, so do not need video capabilities in my stills cameras.
I am thinking of changing systems. To begin, an A-900 body, and 16–35, 24–70, and 85/1.4 lenses. Then there's the question of whether the 70–200/2.8 Apo or the 70–400 Apo is the lens to get (the reports I have seen of the 70–400 are glowing re. sharpness), or whether to go with the 70–200 Apo plus the 1.4 extender; that's long enough for my needs—and I would prefer the wider aperture of this lens.
I know the 135/1.8 is the lens that excites everyone, but if I were to get that, I would need something longer—so some kind of idea of how the 70-200/2.8 Apo compares at a similar focal length would help that decision. In the sometimes very restricted working areas we have on these locations, sneaker zoom (and even changing lenses) can be really restricted, so I would be inclined towards the zoom simply for this reason, if it's even only close to the same IQ.
The pluses for me of the A900 over the D3s are simply the extra MP, and the Zeiss zooms—even though the current Nikkors I own are are very very good. As well, I really like the in-body stabilisation; for me, this offsets the high-ISO capability of the D3s considerably and I like the non-grip form factor.
I have written about the usefulness of Nikon's Live View elsewhere, but others have mentioned that the finder is good enough to see in-out of focus for tilt, and because the 14–24 is so good, I find I use this in preference most of the time anyway, with careful camera placement. The additional pixels of the A900 make this (fine tuning perspective correction in PS) even easier.
The Mirex plus one of the Mamiya or Hasselblad lenses might do this work, although finding a lens wide enough might not be easy. This aspect is the only real unanswered question for me presently. For the occasional table-top work, the Mirex seems very usable.
I could simply get a D3x—but paying a stiff premium over the Sony for the same sensor, more or less, just does not sit well. I like the smaller/lighter form factor of the Sony, I don't need a vertical grip, and the sensor cleaning is a necessity on these sometimes very dusty locations—and the number one reason why I did not get the D3x in the first place when I upgraded from a brace of D700 bodies.
All suggestions (for/against/orthogonal!) gratefully received.