I wouldn't be disappointed if Inu was selected at the back, he's one of my favourite players and I agree he did a pretty good job there (apart from the error, of course). I can also see where you're coming from with Fisiiahi's attitude, however, I think I've seen enough from Fisiiahi last season (at fullback) and in the Vulcans that warrants him a second chance. Obviously, I haven't seen Inu play fullback quite as much as Fisiiahi, but in the brief stints I've seen of him in the Vulcans (won't comment on last nights game as you can't really tell on TV) he has been even more lax on defense than Fisiiahi.
The reason I would chose Fisiiahi over Inu is his support play - players like Matulino, Rapira and Mateo just beg for support play and Fisiiahi definitely has that over Inu. Inu seems to chill out behind the ruck on attack, following the play, then chooses a side for the back door option later in the tackle count. However, my main gripe with Fisiiahi's last performance wasn't his lax defense but his lax support play. I assumed it would be a given that he would be pushing up like Wade V2 and I was sorely disappointed. I'm not sure whether or not I was just not paying attention, but I could only remember a handful of times he pushed up in support; hence I'd be picking him on the assumption that this was a one off thing and not his prescribed game plan.
So in a round about way, I really don't mind who is fullback, I like both players and they each have their pros and cons.