I have today a 1D mk III. But im planning to dedicate myself to some studio pictures / Portraits in the future. I am thinking of 5D mk II / mk III when it comes out.
What is the advantage with full frame vs APS-H when it comes to shooting pictures in a studio
What lenses do you have? You get a little bit more background OOF / Bokeh for the same lens/ framing set-up compared to the 1D3. The FF gives you better options as a walkaround Camera in terms of bokeh/wider framing. I am not sure if in a studio setting you will get much benefit doing to a 5 body from a 1 body.
Maybe better noise on the 5D3. Can't sat much without knowing whats coming with the 5D3 either. Then again, in a studio, you control the lighting, so ISO bandwidth or low noise are not as important as in an uncontrolled environment like a wedding photographer would sometimes have...
There is probably not enough difference to bother changing bodies.
I used my 1D MK III and 5D MK II in my studio interchangably.
One thing to consider is the long battery life of the 1 series cameras if you use liveview, and I always use live view in my studio, the 5D MK II battery will expire long before the 1D MK III, so you will need to spend $200 for a AC adapter.
My 1D MK III included a AC adapter, but I usually did not bother, since the battery would last for 2 hours of liveview use and a lot more.