The 10MP D200 has a resolution of 3875 X 2592, that makes a (native)300ppi photo quality print of 13" X 8.6". The 6MP Pentax has a resolution of 3008 X 2000, that makes a 300ppi print of 10" X 6.7". This may be significant to some people but I don't see it being enough to upgrade on resolution alone. 12MP would give 4256 x 2848 and result in a 300ppi resolution of 14.2" X 9.5", that is more significant.
Remember it would take four times the resolution to double the dimensions of the print size.
I would go for a 10MP Pentax only if it had a lot more to offer, like SR (since I currently own a DL).