I'll also add that, after having running various Mugen's over the years, and now having a Losi, the Mugens require less maintenance and just about 0 repairs. They just don't break. The Losi's, until recently, suffered from bad shock O-rings and weak rear outer hinge pins, and are more sensitive to throttle/brake linkage setup.
But, like every 'car A vs car B' arguement that's ever come up on a forum, it's a matter of
a)what you can get part for from your local LHS.
b)what the fast guys at your track are running, so you can get setup help.
c)what suites your particular driving style.
Why did I get a Losi? Well, it was the flavor of the month at the time, and was hyped to be the only car worth having. Also, I could not find any used MBX5R's for sale when I was looking.