I would go ahead and replace both.

I've had pretty good luck with cheap ebay HID bulbs. The last set I bought was like $28/pair (6000k) and lasted about 3 years.

This time I went with a $10 pair of 6000k bulbs. Let's see how long these last.

They're now $15/pair on ebay:

6000K 8000K 10000K D2S/D2R/D2C HID Replacement Bulb | eBay