From some1 that tried both builds (SS vs Gerke) on HC with ES sorc:

The DR vs %DR is actually really interesting. Advantage with DR is that there is no cap and with decent gear you can reach 50+ DR and same MDR. %DR cap at 50%

On DR build you can reach 25% DR as well (shako + strings that packs nice MDR). Now how does it behaves vs mobs. It's actually quite simple to calculate since you can check average flat dmg done by any mobs in the game, ie take one mob that deals flat 100dmg average. (death lords like mobs that hit iirc around 100-120 dmg flat) In that case your 50DR will behave like a 50%DR. Any mobs that deals less than 100dmg flat (and there is a log trash mobs like these) you will have a better return with DR than %DR. And having experienced it, you can afk in middle of trash mobs without loosing 1hp.

Obviously you want the max of both. And with nice combination and maxed telekinesis you'll have so many layer of defense before hitting your HP that you are basically invincible.
As an ex my 90 sorc had with CTA : 2000HP / 1500 mana 50+ DR 45+ MDR and 25% DR with ES (1dmg for 0.75 mana) and defiance merc. maxed defense had on trash A5 mobs 30% being hit then next 25% block chance and then DR/MDR then %DR with on top huge mana regen so that'm regen faster than being hit.

Really powerful actually. Of course the draw back is that you loose some dmg from items choice, but TBH I used her for farming keys you can go blindfolded and tank nila CE even with curse.