There is no limit that I know of to power.
Attack +6 is better for lower power characters. They will gain more from it. Higher power characters gain more from the courage but you have to remember that courage is only on your turn. Think of it this way. Hugo's ability is ALWAYS active as well. Defense and offense. Ambres is only available on offense. I feel she is a good way to get Killing blows across.
In addition, people suspect you will focus on attacking, when you have Ambre, since Courage only affects offensive characters.
so, you can shake things up, by playing your pillz more defensively.
I prefer Hugo because he has higher power/dmg and he works all 4 rounds of the game. As an opponent of Bridget you just save your pillz when defending, with Hugo you have to waste at least 2 pills throughout the entire match (if you want to win with 2 successful attacks).