Hi there.
I have been experimenting with the various brands in order to change my foglight bulbs on my S3 (H7 Fitment). I have the xenons and have changed my sidelights to the PIAA 1W (TeraLED) which are awesome and don't compare to any inferior LED bulb assembly. These 501 bulbs actually require a seperate ballast as they are that powerful. As for replacement fillament bulbs, I have gone thru Philips Bluevision, Some ebay H7 "Xenon" effect bulbs and a set from Autobulbs Direct. In my opion, they're all pretty similar and if anything the cheapest bulbs (Ebay) actually provide the cleanest white light and look good in the reflectors. For actual quality, you can't beat PIAA, the amount of R&D they invest in producing these bulbs is amazing and in my opion justify the (very) high price. The PIAA Xtreme white are fantastic and as long as you remember that no bulb get get you close to the effect gas discharge bulbs provide, these are a very good second choice. I've looked at aftermarket Xenon's for my fog which would aesthetically provide the symettery for all my bulbs. Unfortunately, there is a reason that they are not fitted as OEM as they take a while to get to full operating temperature (and foglights require immediate response) and require both a washer system and auto levelling system to be UK road legal. Hope this is of assistance. Ants