I know your dilemma. I still have both Nikon and Sony gear which I use for different applications.
Prior to that it was all Canon. Like you, I never could take to the 5D or 5D-MKII ... which was a real shame.
Compared to the Sony A900, the D700 has the higher ISO advantage. It also has an advantage in operating/AF speed and use of flash. And like you, I love the ergonomics and handling of the D700.
However, the Sony has twice the meg count, in-camera anti-shake ... and some spectacular Zeiss optics. The feel and interface is definitely different compared to the Nikon, and you should go handle one before making your mind up.
What interests me a lot is the next generation of the D700. If it follows the lead of the D300 replacement, it will have 2 memory card slots that capture to two cards at once ... which the current Sony's do not have. Big security advantage for one chance, irreplacable captures.
I also used the Canon 85/1.2L prior to switching to Nikon/Sony. Unfortunately nothing quite replaces the Canon 85/1.2L or the 50/1.2L and 35/1.4L ... and I still miss those specific lenses. The Sony/Zeiss 85/1.4 exhibits more CA and purple fringing and doesn't control flare quite as well compared to the Canon L. That said, the Zeiss 85/1.4 can produce a phenomenal 3D effect, typical Zeiss color and contrast, and lovely Bokeh ... and unlike Nikon or Canon 85s is stabilized ... like every Sony lens is on their cameras.
The Sony/Zeiss 135/1.8 is in a class of its own, and I like it even over the Canon 135/2L which is saying a lot since that's one of the best 135s ever made by anyone as touted in almost every report published. IMO, the Zeiss 135/1.8 beats it hands down for color and 3D effect. I am pretty sure you cannot use an extender on the Sony 135/1.8.
Sony doesn't have a Macro the equal to the new Nikon 105/2.8 ED AF-S VR. If you shoot table top food, you should also know that Sony doesn't currently offer any T/S macro lens. Nikon's new optically reformulated PC-E 85/2.8D T/S Micro Nikkor is in a class of its own for this application.
I have the Sony 70-200/2.8G ... which is an APO zoom with 4 ED elements BTW. It is not a Zeiss lens but frankly you'd never know it. It is quite good. Most certainly better than the aging Nikon version ... but that may change with the recent announcement of Nikon's replacement with improved nano coated ED optics and next generation VR ... which unfortunately will come at a hefty price tag.
I am taking the Sony 70-200G and Zeiss 16-36/2.8 to the Detroit "Dream Cruise" today ... the world's largest moving car show and exhibit of classic and hot rod vehicles. I'll try to post some shots from the G lens with and with-out the 1.4X extender so you can see some of what it can do.
-Marc