GalaxyMap asks: parsec or lightyear. - and why?
Light's time-of-travel goes toward communication across relativistic space. This is most-practical for the systems nearest each other. Traditionally we've favoured Alpha-Centauri and Sol.
Where GalaxyMap hits a nerve is that the parsec has its own arguments-in-favour, for near distances. The parsec is based on parallax in milliarcseconds, against our Earth's (half)year. It's most-accurate, closest-to. Once we lose reliance upon lightspeed communications (that is: we're on the Enterprise, or an Imperial Star Destroyer) then it would seem that the lightyear obsolesces. The parsec becomes king.
As a hard-SF converso (no marrano!) I prefer the lightyear. I like to imagine communicating with the Belter colonies around ε Eridani (and not 40/ο Eridani) and 55 Cancri; or at least with the ark-ship on the way thither. But by that same token we're still stuck on one planet such that I prefer SF at interplanetary levels.
So mostly I'm still working in astronomical-units. Once we're off the 0.7-5.3 AU range I suggest, for communications, the lighthour. This goes up to Neptune's resonant kingdom in the inner Kuiper. Scatter'disc could go to the lightday.
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