The Max Level Nightmare at 11.7$ feels like a steal, and I actually bought another one as it is my favorite card in the entire game.
The Pegasus & Constantia are at least 3x as expensive and I would say those have a worse risk/reward ratio. Imo, you are better off getting 3 Nightmares as the times these 2 other cards make the difference to keep it alive doesn't feel that high and they can be replaced by other monsters more easily.