In general, a speedlight is a wear item: every flash tube discharge and capacitor charge cycle contributes some wear, so a unit used every day is more likely to wear out sooner than one only exercised occasionally.

That said, electronics also don’t always benefit from sitting unused forever. Periodic use can help confirm the unit still works and may be better than leaving it untouched for many years. So the usual practical approach is:

• store it properly,
• remove batteries for long-term storage,
• exercise it occasionally,
• and use it when you need it rather than trying to “save” it indefinitely.

Real-world longevity varies a lot. Some flashes last for decades, while others fail earlier, and accidental damage often matters more than careful rationing of use.

So between your two examples, the lightly exercised stored unit would likely last longer in total firing life, but the difference may not be meaningful in practice. The simplest advice is to keep the SB-800 and use it until it dies, rather than buying a replacement solely to preserve it.