I'm firmly in the Saras is decent camp. She has some uses and is more helpful when you have a stronger team trying to get even stronger, but not particularly amazing herself, so can't really carry the day or anything. Eventually, you'll get to a certain point where her heal and protection buff is enough to cover anyone from dying, and you can transition into a more offensive team. That's where Saras starts becoming good, while Nike just keeps falling further behind (even with the new buff she just got - don't get me wrong, it makes her better, but it doesn't really fix the main issue).

That said, I'd still say don't use an mtix on Saras unless you know for sure you're going to be maining water (have rudra, will pick up vohlu later, etc...). I'm sure there's a better pick out there.