Please help me choose my next affordable fulll frame

Thank you in advance for your help,

I am a professional photographer dedicated to Headshots ( Modern Portrait Photography), its time to hang my 5D and purchase a new camera.  I have three considerations and am looking for your feedback and help to determine my next direction. My considerations are the Canon 6D, Nikon D600 and Sony SLT-A99.  I seem to love all three and found some personal advantages and disadvantages for all three, here are my thoughts but am looking forward to your input;

Headshot Photographer wanting new gear, Canon 6D, Sony A99 or Nikon D600

Canon 6 D,

Advantages,

I already own a Canon and have lenses for the Camera.

ISO to 25,600 with expansion ( I am not sure exactly what that means... Hopefully somebody can explain what expansion means?) to a crazy 102,400 ISO.  I am excited to test this ability - WOW.

Time Lapse Recording - needs a computer.

Disadvantages,

Video records in mono and does not allow stereo  microphone ( I really don't understand, why?)

I have never really liked the ergonometric feel of a canon and miss the Nikon's feel and touch.

Don't like the Canon's menus.

Nikon D600

Advantages,

I am looking forward to going back to Nikon - I ditched my nikon when I switched from film to digital.

Ergonomic feel and screen menus are great

It was highly reviewed in DPREVIEW ( I would like to know what are the other advantages over my two contenders).

Best Time Lapse recording features of the three

Disadvantages,

ISO is to 64,000 with boost to 25,600 ( Anybody know the difference between boost and expansion?  Why are they classified as so? )

Video does not allow for stero

SONY SLT - A99

Advantages,

Sony is a late player battling to be noticed and will offer more features.

Its the flag ship camera with the same senson as much more expensive Nikon's ( somebody confirm this rumor?)

Outstanding video capabilities with auto focusing.  One must take Sony technology seriously as they have been a leader in technology for video capture.

Genuine description of ISO abilities, does not have an expansion or boost feature which I find misleading?

Disadvantages,

$700 to $800 more expensive than the Nikon and Canon version.

No time lapse feature

Thanks in advance,

Carlos

The following are samples of my work.

Headshots of Kevin photographed in my portrait studio	Modelling headshot captured during a session.
Actor's Headshot of Michelle outside my home studio.	DA of top talent a corporate headshot photographed in my home studio

Extra Notes,

Would anybody care to explain what this ISO expansion means?