They are definitely very different. I can mostly give you a comparison considereing both are EQ'd to the Harman target. What persists is the 600 being the neutral king with an intimate soundstage, i.e., narrow. Bass is obviously still rather lacking, but that doesn't bother me too much. The Sundara on the other hand feel more energetic, have much better instrumental separation due to the planar technology and a small to medium wider soundstage compared to the 600. To my ears, the Sundara have an artificial feel to them in their sound which isn't necessarily a bad thing, but it can feel fatiguing after a while. I switch between them pretty frequently but usually end up sticking with the 600 for longer periods of time, in part due to the more comfortable fit but also they are the better "allrounder" for me and generally feel better suited for daily use. The Sundara work really well for some intense listening sessions, especially with more energetic/electronic music, orchestral music and the like.