I've run several of both. I find the MT21 to be much quieter and longer lasting on pavement.......grip seems to be similar and I ride my KLR's in a "spirited" manner, on and off road. ....I should clarify that by "longer lasting", I mean I run the MT21 longer......I tend to toss the D606 front earlier due to the extreme "braking pattern wear" that it suffers from. All knobs will get that wear but it's worse on a D606 than any other tire I've run. Once that wear starts to set in, pavement performance suffers and I toss 'em. Not sure which one would last longer if you rode them to the cords.....I don't do that.