Stone Elemental Defense (Effective Health)
(Fire) 1737 x4 (75% Fire Res) = 6948
(Other Elemental) 1737 x (1 / 0.38) = ~4571
Note: The exact values will of course change as your Merc levels and gains resists/life.
For Physical Attacks I'm going to use a Doom Knight (Players 1) as a sample monster.
Level: 82 AR: 5929 Damage: 65-139 (Average: 102)
I can't see the exact Defense value and I'm being lazy on calculating it so it looks like say, 5700 from the picture.
Merc is level what.. 83? (Looks close based on resists) I can't quite see, so take it with a grain of salt.
Note: It should be mentioned that as enemy AR or monster level increases/decreases (relative to the Mercenary), the less effective Defense becomes, whereas the increases in health / damage reduction are static.
Some crude calculations (hopefully not erroneous though) but maybe they'll give you some insight. The FHR differences and +Strength are present but don't easily fit into a calculation-- keep them in mind regardless.
Or you can just make Treachery. It's dirt cheap (Shael+Thul+Lem) and beats most armors out there. 45IAS means more DPS means more leech means more survivability. And the Ctc Fade is a no brainer.
If you insist on using either Stone or Shaftstop, I'd say just test it out and see which one keeps him alive longer. That's the point with Sorceresses merc anyway, to stay alive. It also kinda depends on what targets you run. If you're just questing, Stone might be better since it adds resists.
I can agree wholeheartedly with Treachery...even with low def it boosts survivability by a lot. My current setup is Treachery, Vamp Gaze and Reaper's Toll/Insight (depending on what i'm farming). If no Vamp Gaze then anything with decent dual leech works just fine.