I've had the Fast Track Pro and passed it on to a nephew.
Superb unit. Loaded with features. Never let me down but does produce some pops and crackles when stretched to the limits.
Just had a look at the M Box Mini and can't figure out why it costs so much.
Not even in the same league from what I can see but someone who owns it will probably enlighten you as to its strengths.
For my money though.... the Fast Track Pro.