I don't know, if you would've asked me a year or two ago which HID's I like the best, I'd say a bluer temp like 6,000K or higher. But after seeing other cars with HID, just my personal opinion, but the bluer/purpler color temps look terrible. It looks so forced like "Hey, you should be impressed cause I got teh HID!" OEM Bimmers, Audis, Lexus' etc still have a pinch of blueness in them, but especially BMW is more towards the yellow end.
If I'd invest in these at some point I'd rather have function vs. looks, so I'd probably go for 4300K. That shot against the garage between Silverstar halogens and 4300k HID's kind of shocked me how when you compare those technologies side-by-side, even 4300K is a vast improvement. I dunno, just my preferences on the issue.
Now a slightly unrelated question, does anyone ever have problems with the cops in aftermarket HID's? Granted, a new car like the Mazda5 (esp considering it has projector fixtures) may pass the test easier than say, a mid-90's car before HID's were even on the scene.