I would think the best outcome for both Finland and Sweden would be not to join NATO as this will wreck their relationship with Russia.
Probably not as much as being invaded by Russia though, and there is a precedent in history for Russia annexing part of Finland, and in more recent history of Putin doing everything he can to recreate the Russian/Soviet empire. It is hardly surprising that the Finns (and Russia's other neighbours) do not trust the Russians with Putin in charge.
He is a danger to world peace and although I too am opposed in principle to blocs such as NATO, if I lived in Finland I would be very concerned about security.