The single biggest difference between standard definition (SD) and high definition (HD) or IP CCTV surveillance systems is the resolution (quality and size) of the footage and images recorded.
The maximum resolution that an analog surveillance system can provide is limited to D1 resolution which is 720X576 pixels (PAL) or 0.4 megapixel. Even if a higher resolution camera (such as a 700 TVL camera with a 1020X596 pixel resolution) is used after the video signal has been digitized in a digital video recorder it will only display and record at D1 resolution.
The quality of the video recorded on an analog system is further reduced by converting the digital camera image to analog then back to digital. This creates a loss in overall picture quality when comparing an IP based system and an analog system of the same resolution eg 4CIF or D1. You can see in the comparison below that the IP camera produces a better looking picture with more detail even though (according to the manufacturer specs) the image sensors produce the same resolution image.
In comparison even a 1.3 megapixel IP camera gives a resolution of 1280X960 pixels which is more than three times the resolution than can be achieved from an analog CCTV system. Network cameras with 2, 3, 4 and 5MP resolution are common now with even higher resolutions being achieved all the time. A network video recorder (NVR) can display and record whatever resolution image is provided by the IP camera.
So how does this translate to what you will actually see from recorded footage?
This image shows the difference in image size and quality from analogue cameras going into a Digital Video Recorder (CIF and 4CIF) and IP cameras going into a Network Video Recorder (1.3 and 3 megapixel). Note that not only is the image size different but when the image is zoomed there is a lot less pixel definition with the CIF image (see the yellow boxes).
So in conclusion you will in most cases get a superior image using an IP camera system over an analog camera system. At present however, IP is still more expensive than analog even at similar resolutions so you must take into account your available budget especially if your requirements are for a system with a high camera count. If on the other hand you have a large open plan warehouse or car park to cover then you could reduce the number of cameras required by installing a high resolution system so the overall cost will be comparable to analog. HDCVI may also offer an compromise between cost and image quality.