Good luck with your new Nikon. Sony doesn't make 12MP FF cameras. As
for why most pros use Nikon and Canon, well, duh. Sony is new to DSLR
business. Most pros are already invested in Nikon and Canon. They are
not going to sell everything and switch brands, especially when Sony
doesn't even have a pro DSLR that competes against D3 and 1DIV.
As for A900/A850, I stand by what I posted earlier. A900 will outperform
D700 at ISO 400 and lower. AND, if you resize A900’s 25 MP to 12 MP
(same size as D700), or print them equal size, the difference won't be
noticeable till ISO 1600 (or maybe even 3200).
Nikon D300 was widely regarded as the best high ISO APSC model.
However, if your resize A900 images to 12 MP (or print them equal size),
A900 will easily outperform D300
(the following comes from dpr forum, but originally imaging-resources)
A850 images were resized to the same size as Nikon D300