Personally I use 5/3/5 over 5/5/3 because while Mistress is great, when I do use it it'll be several turns befroe there's enough environmental tiles for Mistress to be worth using again, and by then I'd have already generated 7+ reds. So I dont really have a good reason to put Mistress at 5 and I get the exact same result at 3. so I'd rather buff her black so that it'd fill the board and tac on 300 damage ever turn. you say they conflict, but damage is damage, you have to match tiles anyway so there's no reason not to use black when you can, especially when 2 uses fills the entire board with strike tiles.
People that favour 5-3-5 probably play against lower level enemies where making an extra turn doesn't matter much. With tougher enemies, a cheaper board-shuffling ability becomes life and death.
Please try out the 5-5-3 build. Her covers are abundant anyway, if you're unhappy, you can go back to 5-3-5.
Easily the best, if not in the top 3 1* Characters.
She is, for the cost(AP & IRL), one of the best board manipulators in the game. Esp. In this post-Thor-nerf MPQ crazy world. If she is in your squad, the board should NEVER reset itself. EVER.
Also, most other posters are right. 5 in red is unncessary.
Or, at least, they were right. Now that R48 happened and we are all stuck in this piecemeal-boosting MPQ world, having MotE go off at 5AP might be worth reconsidering. The main idea being that you fire off MotE after a single regular Red match.
So anyway, yeah. More <3s for ModStorm!