theblakester was mostly right but made a couple small math errors.
Many people think that 640 resolution is double that of 320 resolution (I know, Pulsar uses a 384 core, but I will use 320 here just as an example). It isn't. It is quadruple the resolution. Remember that we are dealing with a two-dimensional grid. For example, is a 2x2 square double the size of a 1x1 square? No, it is quadruple the size. That is what we are dealing with on the resolution front. So 640 resolution isn't double 320, it is four times as much.
Maybe it would be easier to just consider this in the actual number of pixels...
Using the actual scopes in question...
The XP50 has 1.6x native magnification at 640x480 resolution. This gives you 307200 pixels
The XQ50 has 2.7 native magnification at 384x288 resolution. This gives you 110592 pixels.
So the XP50 actually has 2.78 times the resolution of the XQ50, but the XQ50 has 1.69 times the magnification at native magnification. This is true, but is misleading and isn't the whole story.
People like to run the numbers by zoom level, but that just isn't relative...
Native
XQ50 2.7x 384x288
XP50 1.6x 640x480
2x Digital Zoom
XQ50 5.4x 192x144
XP50 3.2x 320x240
4x Digital Zoom
XQ50 10.8x 96x72
XP50 6.4x 160x120
8x Digital Zoom
XQ50 NO 8x digital zoom
XP50 12.8x 80x60
Probably a more comparable way to run the numbers mixes the magnification levels. Unless people are shooting very close, the usually will not be using the 1.6x native magnification on the XP50. They are going to bump that up, often double to 3.2x. They will get more magnification, less resolution, and a more narrow FOV, but be able to make a more precise shot with the reticle covering much less of the target. For comparative consideration, this is similar to the base magnification and resolution of the XQ50, but somewhat better. Here, what doesn't matter is how many times you zoomed, but what the actual level of magnification is.
XP50 1.6x 640x480 Native - Low Magnification, larger FOV, best level for scanning. (307200 pixels in use)
XQ50 2.7x 384x288 Native (110592 pixels in use)
XP50 3.2x 320x240 Zoomed 2x (78600 pixels in use)
XQ50 5.4x 192x144 Zoomed 2x (27648 pixels in use)
XP50 6.4x 160x120 Zoomed 4x (19200 pixels in use)
XQ50 10.8x 96x72 Zoomed 4x (6912 pixels in use)
XP50 12.8x 80x60 Zoomed 8x (4800 pixels in use)
This does not tell us about image quality, however. So I took the number of pixels in use and divided it by the level of magnification and came up with the number of pixels relative to magnification level...
XP50 1.6x 640x480 - Low Magnification, larger FOV, best level for scanning. (192000 pixels used relative to magnification)
XQ50 2.7x 384x288 Native (40960 pixels used relative to magnification)
XP 50 3.2x 320x240 Zoomed 2x (24000 pixels used relative to magnification)
XQ50 5.4x 192x144 Zoomed 2x (5120 pixels used relative to magnification)
XP 50 6.4x 160x120 Zoomed 4x (3000 pixels used relative to magnification)
XQ50 10.8x 96x72 Zoomed 4x (640 pixels used relative to magnification)
XP 50 12.8x 80x60 Zoomed 8x (375 pixels used relative to magnification)
What does this mean? Well the XQ50 at 10.8x, for example, is going to have an image that looks a little better than the XP50 at 12.8x because the XQ50 has less magnification and more pixels in used, BUT LESS MAGNIFICATION MEANS A SMALLER IMAGE.
So let's make the calculations even more relative. I think I have this right. Somebody with higher conceptual math skills may be able to come along and say I did this completely wrong, but I believe this is correct.
When we look at the comparable levels of magnification for the basic zoom levels, the XP50 is always has 1.185185185185.... (repeating) times more magnification. So the XP50 6.4x is 1.185... times more magnfication than XQ50 5.4x. Since resolution is squared, we square 1.185... and multiply it by the resolution (pixel count) of the XP50 to show the comparable number of pixels in use at the XQ50's magnification. Make sense?
So at 2.7x, the XQ50 has 110592 pixels in use and the XP50 has 110407 pixels in use. Pretty darned close, right?
At 5.4x XQ50 has 27648 pixels in use and the XP50 has 26970
At 10.8x XQ50 has 6912 pixels in use and the XP50 has 6742
So basically, the XQ50 actually does have a better image than the XP50 at the same levels of magnification, but the difference is very slight and most folks probably would not be able to tell the difference. For all practical purposes, the picture quality is going to be the same, given the same level of actual magnification.
So what do you get for the extra money of for the XP50? You get a lower native magnification with a higher resolution and larger FOV on the low end and a 1.185... times more magnification at the high end. Where the levels of magnification match, you are getting the same basic image quality.
If you do a lot of shorter range shooting inside 50 yards and need the wider FOV, or need it for scanning, get the XP50. If you do a lot of shooting beyond 75 yards, I think you might as well go with the XQ50 because everything from 2.7 (native) to 10.8 (digital zoom) magnification is going to be the same basic image as the XP50. The XP50 does go to 12.8x digital zoom, but the image is getting pretty pixelated at that point.
{WHEW!}