I am not sure how those 2 are different as per your definition, but I think I like BOTH:
My definitions are just the same as everyone else's definitions: pixel density = pixels per inch, and pixel count = the number of pixels. If I want a high pixel count but a reasonably low pixel density, then I get a bunch of huge giant desktop monitors. For example, on my desk at work, I have seven 1920x1200 monitors and one 2560x1600 monitor, and every now and then I actually use all eight at once!
So, what do I do while on the road, when I only have an ultraportable laptop? Well, I just avoid doing any serious work that requires me to view many windows simultaneously or to view hundreds of Excel columns.
- Pixel Density - Sharper resolution to draw the images on screen.
- Pixel Count - More working room/ space.
For viewing photos, high pixel densities are better of course -- no argument there. But as long as we are stuck with Windows' current primitive scaling capabilities, high pixel densities would also make text and icons look annoyingly small. IMO, for 16:9 laptops running Windows, 145-155 DPI provides the best balance between real estate and viewing comfort. That means:
10.1" @ 1366x768 (155 DPI)
12.5" @ 1600X900 (147 DPI)
13.1" @ 1680x945 (147 DPI)
14.5" @ 1920x1080 (152 DPI)
15.6" @ 2048X1152 (151 DPI)
Unless Microsoft comes up with a better scaling method for Windows, I don't think it makes sense to introduce PC laptops with >155 DPI. I played with Windows 8 Beta briefly and I don't think its scaling is any better than 7's. As you probably remember, I used to own three Thinkpads with 15.0" QXGA. They were good for bragging but awful for actual use. When increasing pixel density above around 155 DPI, productivity actually declines because things become so uncomfortable to view. Have you tried the Sony P Series netbook, which has a "retina" display with 222 DPI? I tried one at Frys and had to put the screen less than a foot from my eyes.