After hearing lots of "arguments" for and against socketing avoidance / stamina I decided to write my own simulation tool to see what gem gives the highest survivability.
Results first:

Stamina wins

so my gut feeling was right, hehe.
The tool simulates a boss fight against hardhitting Bosses like Gruul / Magtheridon.
The Boss has a 2.0 second swing timer with 6500 BaseDmg (after Armor) and cleaves for 1.5*BaseDmg every 11 seconds.
The combattable is just plain avoid or hit (unmodified at 0.5 avoidance).
Heal is modelled as 2 full Stacks of Lifebloom ticking every second for 1.8k, a 4.5k greaterheal every 3 seconds and a flash heal every 2 seconds. Both of the last two heals have a 10% chance of not being cast due to AoE Silence / Brainlag whatever.
The MT has 18.2k HP and the fight continues for 10 minutes.
It is evaluated over 200.000 tries for each socketing option(15 slots assumed), and the output is the percentage of wipes.
Any comments would be appreciated.
And thanks again ej for hosting such a nice forum, I gained a lot of insights reading it.
Bye Berthold
p.s. If anyone is interested in the code I can send the VC++ project.