The Bruko tone is just a bit thinner, less low end than the Martin. Still a wonderful tone though, very clear. The Martin action is a bit lower, very playable, as was the Bamboo I sent back. The Bruko action, with 1 piece bridge, is higher at the 12th fret, as many have noted. That's OK for me, right now. The Bruko is also a bit louder.
IMO, the Martin is basically just an S-1, with a premium price for the Centennial label, black tuners, black nut & saddle, and Martin logo on the back of the headstock. To me, the Bruko woods seem to be much higher quality looking, the Martin wood just average. Not sure what it is, but I just love playing the Bruko, compared to the Martin. Some ukes just grab you.