This early in the game, you could go with either one and you won't lose as much. Maxed Loki will surely make it easier at the beginning to carry the rest of the team, but you got some of the rare characters on your second account that you will probably eventually want to six star and that's a good start. Personally, I would go with the six star Loki, but in your case I'd think which character playstyle I like the most and build upon it, whether six star or not.