Here are my recommendations for recordings of Schubert's 8th (or 7th),
I. On modern instruments,
IMO, you should definitely try to hear the great Staatskapelle Dresden play the 8th, either in recordings by Wolfgang Sawallisch (from the 1960s), Herbert Blomstedt (from the 1970s), or Sir Colin Davis (from the digital era). Of the three, I prefer the Sawallisch & Blomstedt recordings myself (but like all three),
Sawallisch:
Blomstedt:
I've also liked Otmar Suitner's recording of the 8th with the Staatskapelle Berlin, on Denon,
Finally, Eugen Jochum is likewise very fine in the 8th, as well, with the Boston Symphony Orchestra on DG. Interestingly, Jochum once told a friend of mine that his Schubert recordings were the ones that he was most proud of in his career (which makes sense, considering how fine Jochum was in Bruckner, a composer that was obsessed with Schubert),
II. I'd also suggest hearing the HIP treatment on modern instruments that the symphony gets from Nikolaus Harnoncourt, either with the Royal Concertgebouw Orchestra, or the Berlin Philharmonic (I only know the former, but the Berlin recordings get excellent reviews, too).
III. Lastly, you should definitely try to hear some of the excellent period instrument recordings, & especially from Bruno Weil & The Classical Band, but also conductors Jos van Immerseel & Anima Eterna Brugge, and Frans Bruggen and The Orchestra of the 18th Century, as well. Interestingly, of the three conductors, only Immerseel took the pains to find and use authentic Viennese horns, which are very interesting to hear in Schubert, since they can make his symphonies sound more influenced by Beethoven than Haydn or Mozart, which of course puts them into a later context, where they belong.
Weil:
Schubert: symphony no. 7 ("Unfinished"), Bruno Weil, The Classical Band
Immerseel:
Sinfonie No. 8 in B Minor, D. 759 "Die Unvollendete": I. Allegro moderato
Bruggen:
Schubert - Symphony No. 8 'Unfinished'