In a thread I created in August last year, I asked the community if it would be cool maxing a 2nd Gazelle and a 2nd Joon and his costume. Everybody said YES. So I proceeded to max my 2nd Gazelle but was never able to spend darts on 2nd Joon & costume, although I left both versions of the 2nd Joon at 3/70, ready to ascend if I want to. Somehow, even with 22 available darts currently in my inventory, I still haven’t pulled the trigger fully ascending 2nd Joon. Reason? I like my AOE heroes (Drake, Onatel and Rana) even more than my snipers (Poseidon and costumed Joon) as the former reduce more total HP from the enemies than the latter. I relegated my snipers on my 2nd monoyellow team in wars while my primary monoyellow team is mostly composed of those AOE heroes, even in raids and tourneys.
Having said that, if the OP’s question is what I have to resolve with the current roster I have, I would opt for Thor by reason that he is very fast.
But it really depends on your playstyle and availability of heroes maxed. Don’t get me wrong Joon and costume is a very good hero. Unfortunately, I just am not really a fan of snipers. I prefer hit-3 heroes