Come on man. It is a very tough question because it is so diverse.
Do you plan on racing?
The buggies are more nimble than the truggies. Truggies are easier to drive.
Everyone is racing buggies. Only some are racing Truggies. Buggy racing tends to be more serious. Truggie or Monster Truck seems to be more laid back.
The Losi is more agile and the Mugen is more stable out of the box.
The Losi is agile because it is light, has centerline weight distribution, very effective gearing that uses power with ease. The Mugen car is much heavier, wider weight distribution so it has more body roll. Now you can work with either of these designs to move them towards the other to a certain point. But for the most part that is how they are.
I will not take anything away from either brand.
I am personally running a Losi Eight.
I like the class because it is serious. I think buggies are the cream of the crop in offroad circles. They are ballistic with minimal horsepower. Every race night there is a class where I am at. There is not always a Truggy class. I have owned both cars and each is as durable as the other. I prefer the Losi's agressive characteristics to the Mugen's more forgiving handeling.
This is a good base line and what you are looking for is completely up to you.