At web size you won't notice much difference...
K-3
K-5
The AF is better for screw drive... the first picture taken with the Tamron 90 macro, has very fast AF, the SDm, is pretty lame. Made worse by the comparison with your screw drive lenses.
I like the AF system... but I don't feel I have to have it, and cropping leaves you larger files. The K-3 is like the luxury version of the K-5.
But it's nothing like the old days.. when I went K20D to K-5 every image was like "wow, wow,wow" and I find it hard to look at most of my K20D images now... it's not the same with the K-3, my K-5 images still hold up nicely.