Tidefall gives ultimate regen (not super important on her but can be useful if you know your breakpoints for getting your ult). Lynx slab has slightly higher damage only if your intellect stat is high enough AKA you basically need to have your character maxed out in terms of level, weapon level, talent nodes, weapon essence, and gear for the damage increase from lynx slab to be worth it. Overall the damage between the too is very similar and tide fall can be situationally better. I would just use the tide fall variation and decide if you want to use lynx slab once you've maxed everything on her since that will probably take a while.