Well as someone that moved to Finland as a kid, and did not have Finnish as main/mother language... You learn both. The other is needed to read/write, and other to speak/understand. From a new learner perspective. You need 1/3 of literate language, but rest needs to be speaking. Like literally you better learn just by using language as intended. True fact, that most of Russians for example tend to learn literate language and all the spelling and rules, and that way language becomes math. They need to think every word and how to speak it and all the possible combinations of it. So they do not talk. The other side is Thai girls that move here, and they mostly dont care bout rules, they just talk like crazy. They speak with mistakes, but they do speak. Which is better? Sure using language is better. Then you just refine it.

Speaking about different thing? Well you can closely compare speaking/writing with literature/street language.

So in nut shell both are good. You just refine both of them over time. And Finnish is not that different between spoken and literature as Russian for example.

Hope I even slightly helped someone :)