I would probably play Navigator, if I had to choose; I've always been a fan of having the character the party feels is essential, and Navigators are just that. Sure, your ship will have a few, but no one will want to lose one. I have a character I wrote up from a concept I had, of a rather high-Fellowship ambassador-like Navigator, engineered to be hot, and practically mutation-free (noticeably, anyway). If I ever get to play a game, she's going to be what I try to play, with a good assortment of Skills, planned future skills, and Navigator ability (written at Rank 2, so if it started higher, she could only get even better.)
If I didn;t play Asteira, I'd probably go Astropath, if the GM didn't allow a more healthy, robust "oh, and he also happens to be a telepath" character. Playing a psyker who didn't have to have Sanctioned Psyker (with so many options I don't want to have to roleplay for downsides to having power), and possibly also Soul Bound, would be nice, if not a slower progression, though the wriieup for AT says nothing of having either trait, so maybe I'm wrong.
As for the Explorator, he irritates me, with his fear of other people's tech. The Imperium could be a great deal further ahead if they were willing to study and incorporate xenotech, such as that found among the Tau. Such gear has no spirits, that I am aware of, so the AM should be able to us e it without fear, or perhaps place machine spirits into it. If I could play a more xeno-obsessed tech-priest, maybe I'd enjoy it, but probably not. Still, I've heard that tech-priests are easy to pimp for combat and shenanigans, so it could be worth it, if it didn't kill the fun.