The north route is just as safe, it's a marked channel the whole way out of the Swedish archipelago, so you'll have a hard time getting into trouble.
These routes are navigated by some of the larger cruiseships in the world. So a sailboat should be safe as long as you can follow the markers.
I do however suggest you stick to the channel, both the Stockholm and Aland Archipelagos are heavy in navigational hazards.
For instance, if you follow the north route, you'll see a large island east of it called Blido, The waters southeast of that island look nice and open, but its a slalom course littred with rocks all the way. I spent my summers growing up on the south end of Blido, and i couldn't tell you how many people sank their boats there.
But the middle Stockholm-Riga and the North Stockholm-Aland routes are clearly marked, mostly wide, and safe navigation.
Like my dad told me a long time ago about the northern route, "If you can't follow that route, Get rid of the boat - you're a danger to others.
