I voted instant for two simple reasons: arbitrary realtime delays flat-out screw over people who have limited time to play, and good gameplay trumps realism.
College students, parents, cops, soldiers, anyone working more than one job: if you're lucky, you've got an hour or two of free time to play, and that's probably not even every day. Say you went bounty hunting last time, but tonight you're on call and can't go fighting in a RES--you just want to hop into Solo and do a few leisurely trade runs, and the nearest ship with any jump range or cargo space is halfway across the bubble. Sorry, you don't get to play today--either spend what little time you have jumping 10 LY at a time to fetch the ship and maybe you'll get to play next time, or waste 50 minutes of your limited free time just sitting there waiting for the transfer. This sort of pointless, frustrating, tedious waste of real-life time is *exactly the kind of problem that ship transfer is supposed to eliminate*.
Moreover, as a general rule game mechanics that force you to just sit there and wait doing nothing aren't gameplay, they are *anti*-gameplay that should be avoided at all costs if there is any other option. Elite has too many of those already, and they are among the aspects of the game that draw the most hate as it is.
Realism is great, but FDev already KNOWS that gameplay takes precedence over that. It's reflected in countless "gamey" areas of Elite's mechanics: instant "FTL" communications and news, realtime remote visualization, cockpits with glass we can see out of, the fact that we can see anything at all when we're in supercruise, not getting crushed in our seats on a 9G planet, the total inability to transfer credits between players or remotely claim a CG reward while mission NPCs can pay you instantly from light-years away... not to mention the fundamentally unscientific gaminess of the entire Engineers expansion, what with the silly concept of "micro-resources" (really? I have to go scavenging on a planet to find a trace amount of nickel or iron, and can't just buy it for half a credit?) or crafting commodities that are "ubiquitous" yet for some reason can't be purchased anywhere in the galaxy, "data" that's somehow impossible to copy in any way, and "engineers" who keep no records of their experiments and can't replicate their own work even once.
Yet out of an entire universe of examples where gameplay trumps realism, some folks are picking *this* as their hill to die on, because muh immershun.
How about this: if your sense of immersion requires a delay before your ship is transferred, *waste your own time sitting there for however long you think is appropriate*. But don't force everyone else to waste *their* time for the sake of *your* immersion.