I am using various AF points all the time to frame my shots, and I haven't had much lowlight AF issues with the K-5. I had AF issues on daylight.
Sad.
But coming from Nikon D90 (Failing to pass to the D300) I am quite content with the AF performance. Said to be not on par with newer Nikons or Canons, but enough for me. Recently I found out that one of my lenses backfocused. I fixed it, maybe the problem is solved for me.
As for passing to the 5D (the bare 5D, as scribed, so I am taking that) the AF system as I know is very, very, very primitive. They only updated the Mk II to the Mk III because of its primitive AF system. So do the math. I think you will not be able to get better AF with that camera. As for the FF, really a bargain. These things are expensive in Turkiye and the 5D is around $800-900 body only, so a bargain here too. With manual lenses, I'd consider that too.
The D800 and D800E are losing their edge in the market with such problems, coupled with Nikon's recent "price update". Nikon is shooting itself in the foot, by the time problem-free D800's come round, the Mk III's may have reclaimed the FF market.
Last edited by Crosshair; 05-08-2012 at 04:02 AM.
Reason: correction