Hi Denis,

First of all i want to make clear ---> I havent booked neither through Finnair nor Japan Airlines yet. We are currently in the process of deciding for flights just like you and i tend to tumble down the rabbit hole and get excessive with my research. Even though my knowledge is theoretical it might help you or at least point you in a direction so i thought i'll share

On Finnair flights there are two things to consider.
First is the seat selection cost which CPH-Flyer already mentioned. JAL is generally free. only cost for special seats like extra legroom might apply

Another thing i stumbled upon when booking directly on the Finnair site, is that their cheapest ticket class in PremEco only include Carry On Bags and no checked in bags. The price difference isnt huge for our flights it would be 40€ per person per direction.
JAL also has their standard 2*23kg included. You might have seen that anyway as it isnt specifically hidden .

Something i couldnt work out as well is the seat selection itself. when booking through finnair you cant make a reservation during the booking process for JAL seats and vice versa. I read something that it might be able to do it later with PNR numbers but some people mentioned they were only able to choose a preference (window aisle) and not a specific seat. As you're travelling with your partner it might be important to sit together and im not sure how that would work out as we havent completed booking and are hesitant because of that.

In our Case i think we might book with JAL just to make sure that seat reservation can happen as soon as possible so we can be "on our own"

Sorry if i couldnt tell you any new things but i wish you a wonderful time in Japan

cheers
Neth