Color temp is the difference. Philips Ultinons burn at 6000K, while regular HIDs burn at 4100K. Physically, Ultinons differ from 4100K capsules by a purple colored plastic/ceramic piece near the glass. Ultinons will have a bluish/purple hue, instead of the pure white that you see with 4100K bulbs. The trade off is that Ultinons have a shorter life expectancy.
I just Got the K2 6000k kit and the comparison between the philips 4100k and the K2 is a joke. The K2 looks so much better it is like the philips kit is stock halogen bulbs. I highly recommend them. I got mine off ebay for $389, so they are cheaper than the philips and look better. Puts off a nice purple color that is not overpowering, but looks rather clean.
There are other things to consider when buying a HID kit. If color is your main concern, then by all means go for whatever kit that gives you the best color the lowest price. However, with cheap kits, there are things that affect the placement of the light. Cheap kits may overlook optics or they are designed poorly and do not have the bulb on the proper axis. If you want nice purple light, but your light isn't being distributed where it should...then that doesn't make much sense at all.
Buyer beware when buying HID kits. There are good, cheap kits, and their are bad, cheap kits.
Please read up on the info regarding Vision/K2 kits. Vision kits came out about a year ago, and have not had the reputation of being the best kits out there. If you search the archives at the Philips lighting forum, you can read up on people's experiences with this kit. Here's something to read.
As for a reputable kit, I would suggest http://www.autolamps-online.com (they have a very knowledgeable person running that site). Top notch in quality, and fair pricing.
The lifetime of an ultinon bulb is shorter than that of a 4100K bulb from what I remember reading or understanding from the Philips forum. I can look for the link to substantiate my comment, however, if I'm wrong, someone correct me.