Paladin Tanking: +Strength vs. + Spell Damage

[Vyndree]

In filling gem slots which is better +Str or +Spell Damage?

Do not, do not, do not EVER consider +str for a TANKING paladin.

Paladins have these important stats for threat generation:
+spell damage
+holy damage (yes, there are a few pieces that exist with +holy damage)

Paladins have these important stats for mitigation:
+block RATING
+armor
+stamina
+block value

Paladins have these important stats for avoidance:
+agility (dodge rating)
+parry

While strength does help by increasing your block, it scales poorly and is one of the weakest form of mitigation. Strength is a warrior stat because it is the warrior's form of threat generation -- it is not a pally's.

Your first priority will be to reach 490 defense. Your second priority will be to reach uncrushability (102.4%) -- this is only important in raids since you will be fighting level 73 mobs -- you may remain crushable in instances/heroics with no problems. Your third priority will be to get enough spelldamage to hold threat off of your dps (typically 300-350 for kara progression geared dps, 400-450 for BT/Hyjal geared). Once these three priorities have been filled, you may then begin to stack more flat stamina such that you can survive the boss's hits, and continue stacking stamina/avoidance as you prefer (I prefer stamina, since I gain mana from heals so too much avoidance is no good when I'm going oom).

I suggest you read up on http://maintankadin.failsafedesign.com -- it has some great pally tanking guides.