How does video quality differ in DSLRS?

My Nikon d3100 is 1080p and so is the Canon 60d (which I'm upgrading to) so my question is, how does video quality differ if they're both 1080p? Megapixels is only about still images right, not video? Is there going to be any difference in quality between the D3100 or the 60D? And could you explain it a little. Sorry, I'm a total noob.

In DSLRs particularly, things get VERY confusing, especially taking into account megapixels. Now you're completely right to say that the megapixel rating is only the number of pixels in a still image, and separate from that is the resolution of the video, which as you say is 1080p on both of your cameras. It gets complicated however, because of the mechanics of how the camera works. I'm going to try and explain as best as possible. IF YOU JUST WANT A SIMPLE ANSWER WITHOUT EXPLANATION, JUMP TO THE FINAL PARAGRAPH.

So there are three main stages in the camera's capability to record both picture and video. The lens, the sensor and the compression. As I explain this, keep in mind the idea of pixels; a pixel is a tiny square that uses electric to change colour. A number of these tiny dots make up an image. If you enlarge an image, it has the same amount of pixels, just each pixel gets bigger. That is why if you blow up your image to a great size, it turns into several large squares of block colour. But that is just electrical images. Before light turns into an electrical image, there are no pixels. We see light, not pixels. There are no pixels and no resolution when we open our eyes and look around. It is a camera's job to convert that light into a grid of pixels.

The light enters the camera and is focused through the lens. At this point, there are still no pixels, it is simply light. The lens has the job of focus, and the better the lens, the more focused. So your little D3100 probably just has the cheap kit lens it came with, which is just a cheap Nikkor thing that probably cost no more than £50 to produce. Because it was cheaply made, the lens components that made it up are not too great. Overall, a cheap lens means a that the image will be a little blurred, and the colour sync may fall a little out. In plain english, it is low quality. Remember, at this point there are still no pixels, so what ever the resolution it simply doesn't matter. The 60d will come with a much more expensive kit lens, so before the light even comes close to being a photograph or video, it is already better quality.

Next up is the sensor. This is an electric panel behind the lens that captures the light and turns it into pixels. In any DSLR camera that is used for both video and photographs, photography is their main purpose, so actually the megapixel reading is quite important. In the D3100, there's a smallish sensor with 14.2megapixels. That means, it takes a picture that has 14, 200, 000 pixels in it. When you use this to record video, the camera still uses the whole sensor, and simply makes the picture with 14, 200, 000 pixels a little bit smaller so it only has 2, 073, 600 pixels (that's what that 1080p number means). But do not make the mistake of just looking at that final number. Think of where it came from. It came from a bigger image that was made smaller (what we call downscaled). Because it's been made smaller, all the detail has been squished down, but that detail still exists. Essentially, the bigger the sensor, the more detail there's to be squished down, and therefore the more detail in the final image. And the more detail, the sharper and crisper the image. The 60D has 18, 000, 000 pixels to start of with. That is a little under 4 million more than the D3100. That is a big difference, so the image will be a huge amount sharper, despite the fact that they both are squished into that 1080p resolution.

The final stage in the process I will not go into. It is far too complex.

So in answer to your question, yes. There will be a difference. It may not be completely noticeable, but to the trained eye, it is very obvious. Think of it like this. HD TV uses the 1080p resolution. Imaging a news cast using those massive cameras that cost upwards of £30, 000. The image they produce is incredible. Now compare that image to your D3100 (which is also 1080p). On the D3100, it is far more blurry, and definitely not as sharp. I hope this helps with your question. If you want to know anything more, let me know.