Well based on the 2 choices,a better way to decide is to go down to the shops and hold the camera in your hands then decide which 1 you feel more comfortable with.The 400D is smaller and lighter and the K10D is heavier and feels more robust,well to me anyway
As for the given choice...I'll take K10D...is a newer model than the 400D, not to mention its additional features too :thumbsup:
The K10D's in body shake reduction alone is a HUGE advantage. Every and any lens mounted enjoys the benefit of shake reduction. The body is weather sealed. Handling wise the K10D or even K100D feels better than the plasticky 400D. With the Pentax, you got backward compatibility with older lenses. Better value for the price imho. If you're starting out even the K100D is an even better option (<$900 with kit lens new).whats the additional features...
You will no going wrong with the 2 leading brands, Nikon & Canon. They are leading for a reason.
IMHO its not just simply following the crowd. The crowd follows something for a reason not just blind following.