Given that sopiri means 'to long for' it sounds like a transitive verb. Shouldn't 'sia edzo' be accusative instead of having a preposition lead in?
I could be 9000% wrong with this, but I think "sopiri pri iu" is closer to the english "to long for someone" and "sopiri iun" is closer to "to miss someone." Could someone explain if I'm kinda getting the right idea?
Looking over the definitions in PIV and examples of the word sopiri in use, I would say there's no practical difference between "sopiri al" and "sopiri -n". Other prepositions are used as well.
BTW, for "to miss", I say "senti la mankon de".
Ah, I would probably use "sopiri" for missing someone very strongly then. Dankon!