[Brightcolours]

I have full frame lenses more than capable of portraits.

 

Actually the thing that is pushing me towards full frame is pixel density.  The DLA for the 70D is f/6.6.  Since I do a lot of outdoor photography often f/8 or higher is my preference.  Also macros are nice to do at f/11 or f/16 with a flash.  The DLA for the 6D is f/10.5 which is a lot more versatile.  Another aspect about pixel density, may just be my own perception.  But looking at sample photos comparing higher density APS-C sensors, for instance Pentax K-5 vs. the newer higher pixel density K-3, I pretty much liked the K-5 better.  And understand the photo comparisons were intended to show the advantage of the K-3, not the K-5.  It's made me wonder if improvements we attribute to higher pixel count are not simply image processor advances, and focus system advances.

 

That said, the 70D image quality is good...but not always consistent.  Especially with exposure.  The same exact shot can vary too much in exposure.  I expect autofocus and exposure might be better handled with the full frame 6D.


I also like the smaller D70 for travel.  So basically I'm after more versatile, more consistent, and better overall IQ if I go to the 6D.

You use f8 for a purpose, right? And that purpose then being a larger DOF? To get the same DOF, you need to stop the FF camera down to f11. 

You will get the same results, also regarding pixel density in relation to IQ/diffraction softening and light gathering. So that is not a good reason to go FF, since you'd get the same results. Same goes for your macro case... Same DOF on FF means stopping down more.

Also, APS-C 1:1 gives a bigger enlargement of the subject in print than FF (due to the crop factor), so max. magnification of the subject in the photo is an advantage for APS-C.

 

About exposure: Cameras can only guess. The 70D measures for a mid tone (or rather: it assumes a mid tone under the metering area) when using partial, spot or central weighted metering. Varying the brightness of what is under the metered area will vary the exposure. Evaluative metering tries to take an educated guess of what the scene might be like, while placing more weight on the area under the used focus point, which it assumes will be the main subject.

 

The 6D has the same metering modes and methodology,d will not offer much different results either.

 

In my opinion, the only real FF advantage is the possibility to use more shallow DOF.

 

On the Sigma cameras, I do not like the AA-less aliasing generator it is. Also the standard too saturated colours with colour shifts in the hues and the extremely low DR, and the overall quirkiness would make me avoid them.

 

You first have to get clear what you like about the photos that make you think FF. Is it the photographer's skill, is it a DOF you can't get with APS-C, is it the art of post processing which makes the results more appealing, or is it the lens used?