That much wind?
Here comes "a local" with another opinion... I must tell you that if the Finnish weather is not going to change dramatically, you won't have that much wind "most of the days". Sorry to be such a spirit spoiler, but this is a fact you have to keep in mind.
I would ask you to get a 8.0 sail no matter what board you end up to choose. I guess if you're very sporty and can sail upwind with no daggerboard, I'd byu Go, but on the other hand Rio M is a great board without daggerboard (tried Rio S on holiday), too. And the daggerboard gives you courage to go to the water in lighter wind days too. If the wind dies, you're not in that much trouble if you have a daggerboard at hand. Or if you're heading to water only over 6 m/s (around 12 knots) wind, you won't have that many sailing days in front of you :-) But I guess, Go is a bit cooler than Rio.
P.S. If you check the general weather forecast to predict the wind conditions on the lake, they always give the maximum possible wind to warn those ones on the boats. There are lot of "holes" in the wind too.