My rule of thumb is to figure out what the appropriate size you think a hand weapon wielding Saurus Warrior block should be given what the point size of the game is. Let this number be x.
Temple Guard Size
Life Slann TG = x + 2 (TG are very well protected)
Light Slann x +3 (TG are fairly well protected)
Heavens or Shadow x +4 (TG are apt to fight weakened enemies)
Metal or Beasts x + 5 (theory hammer, I have not yet used Metal yet and barely used Beasts, there is some battlefield control in these lores but not as much as I'd like)
Fire or Death x + 6 (TG are mostly on their own, the buffs and hexes of these lores are probably best used to help the rest of your army. Death has Soulblight which can help TG a lot but this is counterbalanced by the fact that Death Slann usually have close to short range with nasty enemies quickly to use the sniper spells)
Is the army you are facing renown for artillery? Add an Iron Curse Icon, +2
Are you NOT planning to keep a supporting unit on both flanks? If yes, +3
Do you have no Miscast protection for your Slann at all? If yes, +3.
Do people comment on your Saurus warrior blocks being large? If yes, -3.
Does your Temple Guard have a well-equipped Scar Veteran? If yes, -3
Does your Temple Guard have a well equipped Old Blood? If yes, -5
Are points an issue? If yes, throw out my guidelines altogether.
If I have points left over when making a list but not anywhere close to another unit, I tend to use the extra points on Temple Guard.