This seems as good a time as any to revive the topic, as we're likely to get an answer with the season 7 premiere!
Personally, if I had to pick, I'd say 'Prince', as it corresponds nicely to Celestia and Luna, and distances him from Queen Chrysalis a bit. But I got the distinct impression from the Finale that he was going to be called 'King' - no idea if I just made that up o.0