Ideally, you'd want to change them as a pair because the other one will have colour shifted so you'll have mismatched lights and yes the other bulb will be nearing the end of it's life as well.
The first link is for a D2S / D2R bulbs. While the guy mentions bulbs under $85 are fakes, I am not sure his are genuine Phillips bulbs at his prices.
D2S is for Projector HID headlights while D2R bulbs are for HIDS that still use reflectors like most halogen headlights.
You said you need a D3S bulb. You can try asking them if they carry the D3S bulbs as they claim they do in their ad.
As for the second link, I've never tried the Hyperion bulbs but RPM is a pretty decent looking company for the 1 time that I dealt with them.
But again, I would buy a pair of bulbs and it's nice that they are local and offer a 1 year warranty on the bulb.
But buyer beware as you may be getting what you are paying for as it seems like Phillips D3S bulbs are pretty expensive from TRS.
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