Originally posted by Rondec Ever sensor size/camera is a compromise. If, you had a full frame camera that had the same pixel density as the K3, same frame rate, same size, had SR on the sensor, and cost the same, there would be no question as to which camera to get (assuming you have a couple of 5 TB hard drives sitting around waiting to be filled). But to this point, not all of those things are available and so different people make different decisions. There have been folks who have left Pentax due to lack of full frame and there have been others who bought Pentax because Nikon never released a true D300 sequel.
In theory, there isn't any down side to full frame, but in practice, there are things that low end full frame doesn't do as well as APS-C. As to whether or not someone "needs" full frame is something only they can answer.
Of course it is a compromize ! A friend of mine is looking for a good Camera. As a victim to marketing he saw A7s that has so better high iso performance (he like to take photo in the night). The thing is A7s is just a little better than D600 or base A7.
A7 in fact look very good: FF for the price of K3. And it is very small. So it seems to fit every possible feature box you could find (except an optical view finder of course).
I taken a look at the optical system when I saw that the Zeiss zoom what only f/4. There virtually no echo system for this camera. The best zoom are f/3.5-5.6 or just f/4. There is one f/2.8 prime... and one f/1.8 55mm prime. While I no doubt this is fantastic quality, thoses lenses are quite expensive (800€ each one at least).
In the end the f/4 lens is big and make the camera bulky anyway. no WR. And if you shoot only f/4 max apperture... Well you might do as well in low light with half the isos, an entry level €400 reflex and a €300 f/2.8 tamron.
You could adapt to other lens, but this is expensive if you want AF, and add to the size of the thing.
In the end, Sony made lot of noise speaking of their low res A7s, and while I have no doubt it is a good camera it will be less efficiant in many occasion than most other FF for low light just because there is no fast optics for it anyway. That might not be needed to have f/2.8 but it the same argument as APSC vs FF... But this time it is too FF and this A7r is on the bad side of comparison.