I have compared 5d2 , D700 , A900 and my new K5 many times just before I sold my D700.
Iitially, I thought I got a K5 just to compare it to my Canon, Sony and Nikon , and expected to be disappointed and return it in a month or two, though I was actually becoming very fond of it , its high ISO especially in real life is already better than any of these 3 old gen FFs, its AF is actually very good , I was positively shocked because I read so many reviews complaining about its AF in comparison to Nikon, most importantly as DXO says , the DR of the KR is just incredible about 13.7EV at ISO100 in my own lab test and its color depth and tonal range is also better than that of the D700 and 5Dmk2.
so, just a couple of generations newer tech makes a crop as good or better than a couple of gens older FFs in all areas(except resolution) , maybe it is a good time for all of us to really rethink about why we still want a FF?
I think potentially higher DR, resolution and better DOF control are 3 main reasons why we might want a FF.
High ISO is definitely not the main reason cause the best crop from 2010 is already better than the D700 with more resolution at ISO1600.
And at base ISO the K5 is much better than the D700 in all regards.
Also it is very important to mention that as we take shallower or lesser native DOF at any given f number of FF compared to APS-C into serious consideration , there is no high ISO advantage of FF over APSC at all because f2.8 of APS-C is about identical to F4.5 of FF, this means when you want to get the same DOF for both FF and crop , you must stop down the FF 1.5 stops more and thus , you need to use 1.5 stops higher ISO speed to get the same shutter speed as with the crop.
after using all 4 systems I have come to the point to realize or at least doubt if I can trust any on line review or mag at all?
I think I can't, since imo , Sony and Pentax systems are clearly better than Nikon or Canon system.
After all you must find what best suits your needs yourself and reading as many reviews as possibly you can find is not the right way to do so.
Just trust yourself and if possible better try them out once to draw your own conclusion.
I just prefer Sony and Pentax after having used all 4 extensively.
Taken from http://www.digitalphotographywriter.com/2010/11/sony-a850-vs-canon-5d-mark-ii-vs-nikon.html