All things won't be equal- smaller photosites give you diffraction sooner, but more ability to crop and more detail. Larger photosites have less noise, but obviously there are fewer of them, so graduations will be more granular. You probably won't see a lot of difference up to 8x10, and will have to do a side-by-side comparison above that until you get really big- so the real questions are about what and how you shoot, if you need to crop, etc..
Your best bet is to go to a camera store and shoot both and bring the files home. In-camera B&W isn't going to be as good as any of the multitude of conversion options and you'll want to see how each holds up to whichever option works best for you outside the "Desaturate Image" choice..
You'll also want to see which "shoots" better for you (you might throw in the regular D40 to do an apples-to-apples comparison.) Finally, you'll be upgrading bodies in 2-4 years, so lens selection may be a better criteria than current body since whichever system you buy into will lock you into a lens line..
Both cameras will suit you well. If you're prone to cropping images more than 6mp might be necessary but otherwise more "pixels" are a hassle. They take up space on your hardrive and they slow down your computer when processing..
If you are out putting to computer screens, large TVs or printing on a consumer printer then 6mp is gobs of information..
Well, I'm flummoxed. I want either the Pentax K100 Super or the NikonD40x. They both use the same chip, but one has 6mp and the other10mp. All other things being equal, will I get better resolution(particuarly for B&W prints) with a the smaller-more pixels camera(Nikon) vs. fewer-larger pixels (Pentax) on the same size chip?.
Roughly the same size but The K100d uses the same chip asthe D40 the K10d uses the same as the D40x (at least thats what is often stated)..
All things being equal the larger pixel camera should be better at low light/high ISO the smaller pixel camera should let you crop more or print larger. There is another thread on one of the forums I saw the other day with those two cameras ...someone printed out the images....try a search (if it's working)..
Both are great cameras..
The difference between 6MP and 10MP is not as significant as it sounds: in the fomer case a 12 x 8 inch print will have a linear resolution of 250 pixels per inch; in the latter it will be 323 pixels per inch. You'd only notice the difference with a magnifying glass. If you regularly want to make A3 or bigger prints the extra resolution would be worth it, otherwise you'll just fill up your memory card and HD with bigger files. Plenty of dpr users have said that they get very good poster-sized prints from 6MP camera..
There are many other criteria you could use to base a decision on....
Is image stabilisation in-body, which will work with all lenses, important to you? if so, a point for pentax..
Do you want to do a lot of flash pictures? Nikon has a better flash lineup..
Availability of lenses? Nikon D40/D40x require the AF-S lenses with an in-lens autofocus motor which limits the selection a little.... but then there are some lenses that are only available in Canon/Nikon mounts but not Pentax. A toss-up..
And so on and so on... the MP question is only one small issue to consider, and far from the most important..
They can't be the same sensor - maybe the same manufacturer, but you won't notice the difference in anyway resolution on all but the biggest prints. What you have to look at is the lines of resolution results in dpreview tests to verify the difference..
The noise performance of both is very good anyway so you probably wont notice the difference there..
I would go for the higher megapixel camera just in case you have to print crops or sell the camera later as it will probably have a higher resale value..