It really depends on how you plan to use the lights.
I've had several sets of PIAAs and PIAA bulbs, I currently still have the Ion Crystal bulbs in my OEM fog lights and Xtreme white high beams.
I had a set of 520s with the super white bulbs, I prefer the color/brightness of the Xtreme white bulbs for driving/long range and the yellow ion crystals work better in the fog than the other two.
I have some pictures some where with all the 3 bulbs lit at once to compare colors. I'll try to find it and add it here.