Since you are talking about Fast Speed Mana heroes it will depend on your play style.
A fast speed mana hero normally needs 8 tiles to charge. With mana bonuses you can bring it down to 7. There is no way to consistently get a fast mana hero to charge in 6.
Now think about how you get 8 tiles vs 7 tiles. Both normally require 3 sets o a 3 tile match. Using/making dragons and crystals can get both done in 2 moves as well.
So do you think you going from 8 down to 7 tiles is going to make a impact on how fast you can fire off the specials? We could argue for and against all day long on this subject and never come up with something most agree on.
So if you are the type of player who wants to shave 8 tiles down to 7 then go mana and need much lower level mana troops to do it. If you do not think going from 8 to 7 tiles on a fast hero is going to impact you play style go critical. Then if you really think critical matters to your tile damage put a crit troop on that hero. Otherwise put a ninja troop on it
also consider the other heroes you normally use with these heroes or hope to use in the future. Will shaving a tile off this hero’s special mess up the synergy you have with the other heroes specials and mana speeds or could it become an advantage?