If you already have bits specifically for Nikon fit cameras, buy the D90. If you already have bits that fit Pentax cameras buy the K7.
If you don't have any existing system, then it gets a bit more subjective. If you take pictures of wildlife and/or fast moving sports, then buy the Nikon. Why? Because the continuous AF on Pentax is much poorer and the choice of lenses of over 300mm is much more limited in a Pentax fit.
If sports/wildlife isn't your thing, then by all means get the K7. I have one and am very happy with it. It has all the features you need, without having a load of features you don't need.
Duncan
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