Hello,
Lore of Heavens is ideal for a Lizardmen Army!
With this lore we can cast spells from turn 1. because of the huge ranges, and low casting valuse combined with Rummination make it a great lore for the Slann Mage Priest. C'mon Commet cast on 2 dice?

Besides Heavens is the most fluffwise in my opinion
One more thing:
It may have worked once, but it is probably bottom lore right next to metal
How many games have you played with Lizardmen? Lore Of Metal in one of the best! Here is a list of Pros:
-Low casting value
-Counters to Heavy infantry
-Cheap and Deadly "6" spell
- Enchanted blades make Razordons usefull!:>
The only Con is the Scaly skin spell. but wait, you can always take Plaque of Tepok, instead of the Mystery discipline, thus freeing 35 points
As for Life, when 8 ed was up a Slann on Life in a Tg was soo OP, but now, It is starting to be a Hipster thing once more

A Lone etheral Slann with Death can win you most games. (Feedback scroll, Banehead, and Obsydian loadstone)