Here is a question that has plagued me for some time and that you might be able to answer: I know that Spock/Kirk was the original pairing name and the origin of “slash” as a term, but is “Spirk” still relatively old as a name for the ship?…
Another thing that seems to affect usage of portmanteau ship names (e.g. “Spirk”) is internet location. I really haven’t seen it widely used on AO3, LiveJournal, Teaspoon, etc. (though that may also reflect the sub-areas of those sites where I usually hang out), and the impression I’ve always gotten is that it was mostly used by younger fans with many of the older fans considering it a silly teenager get-off-my-lawn-and-start-referring-to-ships-properly thing. I mostly ever saw it on LiveJournal in the Jack/Ianto - aka Janto - subfandom, which tended to skew pretty young in its demographics. Which all does support the idea that it’s a more recent phenomenon.
Also, as someone above mentions, when using the name1/name2 format, it’s standard to put the most dominant character’s name first — easier to do in that format than in the portmanteau version, where there might be issues of pronunciation/clunkiness that dictate which character’s name is first.
Me, I just use whatever naming format seems to be the socially accepted one for the company I’m in. *shrugs* For me, the ships are the important thing, not the naming conventions.