Both the Murl Engine and the Unity Engine are free for commercial use, so I'm just wondering which one is actually better.

The best answer I can give is go to the web sites for the two engines, download them, try them out, and decide for yourself which one works best for your needs/experience.

The two engines are very different: Murl is an engine framework that requires programming in C/C++, while Unity is a "visual engine" where much of the work is drag-and-drop of components and setting properties/rules along with scripting as needed.

Only you can decide which engine is better for you.

Judging by your question, I would probably say you should go with Unity before you start tackling Murl.

Murl Engine looks very new. They haven't fully listed the features of the engine (does it support shadows, sprites, widgets, etc.).