I don't have an S7, so I wouldn't know the changes they made to the keyboard since the previous models.
I do use Swiftkey, which has a much better predictive algorithm than the default Samsung keyboard, and can also learn words. The good think in using Swiftkey is that you can take your dictionary across devices since you can sync your dictionary to all your devices using Swiftkey (and yes Swiftkey also works on iOS so you can sync dictionaries between say, your iPad and your Android phone, if it comes to that). So if you can't find another person who knows the answer, I suggest you use Swiftkey.
PS: Any keyboard app will give you a warning that it may save your passwords (duh, you type on it). It's a normal warning when setting up a new default app apart from the built in one. Swiftkey however is a very trusted company. It syncs using Google servers and is owned by Microsoft, so it really can't steal anything new from you even if they wanted to. Microsoft already runs your(or your workplace's) PC and Google runs your phone. LOL. I'm not saying that they do.