I'd at least keep a really bad TN panel for web design testing. Actual designing should be done on a decent monitor so it doesn't look bad on other decent monitors.
Getting S-IPS or PVA/MVA/SVA/whateverVA is largely a matter of taste. You can't really get VA panels without overdrive, so they all have some output lag, one frame at minimum (which isn't really noticeable unless you construct test cases for it). There are S-IPS monitors without overdrive, though.
Colour reproduction is absolutely great with S-IPS and decent VA panels; however, colours tend to shift a bit when changing the viewing angle on VA panels, whereas they don't with S-IPS panels (remember, we're talking about a bit here, nowhere as extreme as TN panels; when doing photo work, you'll most likely be sitting directly in front of the monitor anyway; it's more of a concern when having several screens). On the other hand, S-IPS monitors have absolutely horrible black levels and large, dark areas exhibit a nasty colour shift when changing the viewing angle (they turn somewhat violet or orange).
So what should you get? I went for S-IPS panels for several reasons:
- The monitor featuring them was good value (Dell 2007WFP)
- I don't care about power consumption (S-IPS draws considerably more)
- I use two of them, so I can't stand colour shifting; naturally, when using two monitors, you will look at them at an angle, especially with widescreen ones.
- I don't care much about black reproduction.
My recommendation would be to look for tests and reviews of the monitors you're interested in. There are crappy monitors with decent panels, because the manufacturer didn't think enough or made stupid decisions. A good panel can be ruined by firmware-induced colour banding or generally crappy behaviour, so if you're unsure, read reviews and see how both monitors compare. The panel is important, but even if the panel is decent, you still can't buy pretty much anything.
-huha