This is all from my opinion and experience, feel free to tell me i'm an idiot
It's a lot of preference in what lore you take. I thoroughly enjoy using Light on my Slann on occasion since you can make up for Lizardmen's abysmal initiative and boost their WS in one spell, gives you a couple magic missiles to channel through your skink priests and can have a great "what the heck just happened" potential with Briona's timewarp when your skink horde all of a sudden has a potential 24" charge range.
Even better, if you get Briona's timewarp AND the speed of light off on a unit, they have ASF & In 10, so they gain hatred (on a ridiculous WS), so they're not going to miss as they get to re-roll misses (except against HE - dirty HE).
Another good combo is to get Pha's Protection on a unit from the Slann + Iceshard Blizzard on the unit attacking you from one of your skink priests and all of a sudden, that unit has -2 to hit you. That means, they're hitting you on, at least 5's, 6+'s for anything WS 3 or lower.
----- Enough about the joys of light
After playing a solo slann with shadow the other day for the first day, I've learned the joys of that lore too.
A unit of 5 cold one knights was charged by a unit of 9 undead knights w/ a hatred banner & a +1 to hit banner and a wight king - Potentially a lot of pain. We were doing the grand battle thing where everyone does moves, then magic, then shooting, so in the magic phase I hit them with every debuff that lore has. I ended up fighting WS1, In1, Str 1 (+2 from lances), To 1 knights. Had it not been for their hatred and killing blow, I wouldn't have lost a single model.
Okkam's Mindrazor on a horde of skinks or even skirmishers with javelins make for a great "surprise" unit when they realize they're being shot 20 times with strength 6 javelins.
And _very_ important for a solo slann, the lore attribute. When your Slann gets charged (shame on you for letting your slann get charged! I miss the days of "your slann has str what? and HOW many attacks?"), you can cast 1 single spell and all of a sudden, instead of fighting a slann, they're fighting a low point skink priest or a stubborn old-one w/ a 1+ rerollable save and 5+ wsv.
---- Now for Life
The main reason for taking life, as i've seen it, is to get throne of vines off. This just gives you that LITTLE bit of extra protection. The chance that you get a 2nd miscast if you are throwing lots of dice at spells and losing your slann outright is a major danger.
And if your opponent allows you to get that spell ALL the spells in life are dangerous. You're looking at potentially unbeatable blocks of saurus or even skinks. With regeneration, +4 toughness, and taking hits from shield of thorns, it's a unit you don't want to fight all that much. Plus, if you kill less than 7 models, there's a good chance you're seeing them next round again.
It gives defense against shooting, the ability to bring back guys that failed in the defense against shooting and the big spell has the potential to wipe out an entire unit of troops without blinking. Hit a horde of elves, dwarves, humans, marauders, skinks, skaven, brettonians, undead and they're looking at, on average, a 50% loss rate. If you have a 2 slann list and manage to bring them down to Str 1, kiss that unit goodbye.
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I haven't tried beasts, fire is imo just for dealing lots of damage and I prefer the synergy of buffs/debuffs, metal has some potential (if you know you're fighting armor), heavens is a waste imo since you can use your priests for that, and I haven't tried death.
TL;DR - take what you like, they're all good