I started with 6000K HIDs but subsequently found them a bit too blue. They look nice, but they definitely look "aftermarket" (which they are of course, to be fair!).
If you go for 4300/5000K, they look much more like original equipment. You still get the extra brightness, of course... it's just that 5000K looks more pure-white rather than blueish-white.
It's a personal taste thing, so I would suggest seeing them in real life before taking the plunge.
As far as I know, all of the OEM xenon projection headlights use 4300K capsules... it's the projection lenses which cause the blue tinge you see when they're coming towards you. Fitting a 'HID kit' in a reflective headlight unit means you don't get this effect, so people sometimes choose higher colour temps such as 6000K or even 8000K like Danny, to simulate the effect.
Although I liked my 6000K HIDs at first, I've recently changed the bulbs for some 4300K H7Rs (from eBay) and IMHO they look more natural.
You should also know that you'll need to drill 25mm holes in the back cover of each headlight, as part of fitting the kit. It's not difficult but you'll need to borrow a drill and 25mm hole-cutting bit, if you don't have them. (You can buy replacement covers for about £9+vat each from the stealers if you mess it up - so it's no
problem!)
Oh, and there's been a lot of noise around the legality of aftermarket HID kits. Officially, only factory-fitted xenons are allowed. If that worries you, a HID kit probably isn't the right option. (To be 100% legal, you'd have to fit OEM xenons complete with all the auto-levelling hardware etc and the headlight washers in the bumper, too.)