Disregarding anything else, just based on how they will be with their new powers
I’m not sure we know what buffs (if any) will be given to Atamos or Margaret, so it’s hard to answer that question in any way.
I’d say Margaret. 150% hit on three targets at very fast and an 800+ attack stat will really shine with Evelyn/Almur/Buddy defense down buffs. She’ll also be a bit more useful in offensive raids when these costumed snipers start taking over.
Personally I would not ascend any hero based on current beta balance changes. They are still subject to change and I’ve seen some weird last minute curve ball changes happen before.
Save your tonics for Atomos though, but only ascend him once his final form is confirmed. If he stays average speed he goes from being useless to a very decent hero.
I have limited tonics so this decision is a pretty tough one too. I’ll wait for the final release of the balance change, and see if I get any new green 5* hero by then. Now am aiming for the season 3’s heros.