There are many attributes that define how good a monitor is, but the most important ones for graphics work tend to be:
- Colour accuracy
There are others (e.g. response time, refresh, etc. that tend to be relevant more for gamers).
IPS is frequently preferred by graphics professionals because it has superior colour accuracy and contrast to TN (although VA has better contrast still). It frequently also has better colour range (gamut) but much of this is because historically IPS has been used in more expensive, higher-end monitors, and are therefore paired with better components overall.
In terms of the major factors:
The better colour accuracy of IPS comes from three main factors. Firstly, it has a more linear response than TN (and MVA). Secondly, IPS panels have a higher bit depth (8-bit or 10-bit panels). Finally, IPS architecture means it has better contrast and consistency across a range of viewing angles.
The gamut of a monitor describes the range of colours it can display. The main limiting factor of a monitor's gamut is the backlight used. Most TN monitors will only meet sRGB colour space (or worse). While technically there's no reason a wide-gamut backlight cannot be used with TN, the cheaper nature of the screens tends to preclude it. Many IPS displays also have sRGB backlights, but monitors with expanded gamut, e.g. AdobeRGB or larger, tend to be IPS only.
Contrast ratio tends to be primarily a function of the panel technology. I'm specifically excluding "intelligent" or "dynamic" contrast modes here, because they're not static contrast. TN tends to be inferior to IPS for contrast, however some of the better TN panels come close to some of the worse IPS panels.
So what should I buy?
tl;dr: Buy an IPS.
One reason IPS will be superior is because IPS panels tend to be used in professional monitors geared towards graphics professionals. Beside the differences in panel technology, they will also contain more relevant features such as calibration presets and adjustable colourspaces that will be missing from most TNs. You're essentially choosing between a monitor designed for gamers or typical office work, vs. one designed specifically for graphics work.
But how much of a difference is it really?
A lot and very little. YMMV and it depends on your needs. In terms of the differences I mentioned above:
- In terms of response accuracy, TN cannot come close. However, it's also a function of the monitor's input processing and can be improved by external calibration. Yet, some monitors simply cannot reproduce accurate colours despite calibration, due to poorly calibrated hardware response. But on the other hand, if your material isn't going to print, then accurately calibrated colours are less important. For web only material, which is likely to be viewed by members of the public with hugely varying display characteristics, calibrated targets can even be futile. In my personal opinion, this is one of the few areas where a good TN monitor won't be noticeably inferior to an IPS monitor as long as your work isn't requiring high levels of absolute accuracy.
- Linearity can be another issue with TNs, particularly with darker shades. If you are working with imagery with high dynamic range, this can cause certain shades to be imperceptible. Particularly in heavily compressed video, this can lead to an inability to critically scrutinize image quality at extreme ends of the spectrum. Again, this can be improved with calibration but not eliminated entirely. The difference is subtle though, and you are unlikely to notice if you are not specifically looking for it.
- In terms of bit depth, true 10-bit or even 12-bit panels tend to be IPS only. Most TN panels are either 6-bit with FRC (temporal dithering) or 8-bit. A monitor with FRC essentially flicks a pixel between a high and low value very quickly in order to simulate a value inbetween that the hardware cannot statically display, thus emulating a panel of higher bit depth. This can sometimes be seen as a grainy overlay or faint static over even still images, but usually isn't noticeable from a distance. Again, if your work requires critical precision, then this will be a major drawback, if it's gaming videos on Youtube, nobody's likely going to care. Without looking carefully at specific images up close, you're unlikely to notice a difference in day-to-day use.
- Viewing angles are one area where you will notice a big difference. IPS panels have significantly better viewing angles than TN, which is important for graphics work even if you are looking straight on. In particular, larger TN displays can have such difference in angle just from looking at one corner of the screen to the other that the colours become extremely distored even on a flat image. IPS on the other hand, does not suffer from this problem. This image from Lagom.nl's calibration pages gives an idea of an extreme case of what a flat, uniform colour may look like on a large TN panel - if your work requires colour accuracy, you do not want it to look like that!
- In terms of gamut, if you require a wide-gamut display, then TN-based ones practically don't exist. IPS is the only real option. However, if you only require sRGB colourspace, then higher-end TN displays will match this fine. Bear in mind the vast majority of web content is targeted to sRGB, while TV colourspaces are actually a lot larger (e.g. NTSC, Rec 2020 (UHD)).
- When comparing a wide gamut monitor to one that does not have wide gamut side by side, there will be a very obvious difference in the depth and intensity of the primary colours. This is something that can't be compensated for by adjusting settings on a TN monitor - no matter how hard you try. Conversely, a wide gamut monitor without proper calibration will display web-based content in an oversaturated manner, some describe as cartoonish, because web content is not targeted at wide-gamut displays.
Overall, a good quality TN display can be hard to tell from an IPS when looking head on at typical web-type content, however when looking at an image with high contrast, TN's inability to display particularly bright or dark parts of an image accurately will be very obvious to the eye. In terms of colour precision, it is only if you are working with very subtle differences in shades or brightness that need to be accurately reproduced on other mediums that IPS becomes significantly better. Overall, if you're not actually caring about precision, then the biggest difference you'll notice is a generally more washed-out appearance on TN displays.