AF is there for both cameras, but as you ascertained that doesn't always account for camera shake. If you are worried about shake reduction and you like to shoot portraiture, then Nikon's primes should give you some pause since there is no VR (Nikon's shake reduction system) on prime lenses until you get to longer focal lengths. So the fun primes like the 35mm, 50mm, and 85mm have no shake reduction at all. Whereas with Pentax every prime of every focal length automatically has SR. In my opinion, some people who are new to Pentax might not know they have to wait for the SR indicator before taking the photo. Without doing so makes the SR useless.
So moving from Primes to zooms, the story remains the same. Most of Nikon's desirable zooms (i.e., 14-24mm f/2.8 FX, 17-55mm f/2.8 DX, and 24-70mm f/2.8 FX) have no VR plus they are all very expensive. Additionally, the size of Nikon's zoom lenses are large and their lineup is geared more towards a full frame format whereas the Pentax zooms are designed for a crop sensor. The Pentax 16-50mm f2.8, 50-130mm f2.8, and 60-25mm f4 are a bargain compared to their Nikon counterparts.
If you want to see K-5 portraits done by a professional, look no further than Benjamin Bjorn's photostream:
Benjamin Bjorn's Flickr