What are the genres you primarily listen bro. Both are very different in terms of presentation , Sundara has a slightly bright presentation with a fairly wide soundstage whereas the Sennheiser HD600 is more laidback , vocals are surreal and treble is slightly subdued.
Another thing to consider is what do you intend to drive them with , Sundara go about well with quiet a bit of portable sources and majority of the desktop amps. HD600 need a tube amp to do really well, sound bland with the majority of the solid state amps in the starting range. But they scale tremendously as and when you invest in higher end amps and sound phenomenal.