by LoMa
Mousguard is more about surviving the elements and predators. Not really a magic kind of thing. It's about skill, having the right tool or weapon when needed, being able to assist your mouse watch fellows, and accomplishing your mission. You get rewarded for playing to your character's strengths and weaknesses, and their personality.The two games have different kinds of scenarios and goals, with a completely different feel - at least for me and my family. YMMV.
You can pick up an official pdf of Mouse Guard inexpensively from rpg.drivethrustuff.com - be sure to get the base game rule book PLUS the second smaller booklet - the smaller booklet gives mor details, scenarios, and ideas for playing the rpg.
The only thing that really ties these two together - Mouse Guard and Mice n Mystics is that they involve mice and they are scenario driven. But Mice n Mystics has a boardgame feel with a storytelling aspect, while Mouse Guard really is an rpg.
Ti get an idea of the Mouse Guard stories/scenarios, pick up a comic book from amazon or drive thru rpg.