Anita's post is excellent. I'll just add that for St. Petersburg, you either have to take the St. Peter Line from Helsinki, or else get a visa in advance, which adds both cost and hassle (and don't underestimate the hassle). However, while hotels in St. Petersburg can be expensive, everything else there is cheap compared to your other destinations.
One more thing about cost - transit. For getting around Switzerland, you can use either a Swiss Pass or a Half Fare Card, which saves significant money (which is best depends on your EXACT itinerary). In 2014, I bought a 15 day Swiss Pass, under a promotion, for $492; this not only covered all but about 120 CHF of my transit costs (supplements for the Allmendhubel, Schilthornbahn, and Jungfraujoch), but also all but 15 CHF of my musems (supplement for the Swiss Transit Museum in Zurich).
For getting around Scandinavia, you have to use international trains, boats, or planes, and these can add up fast. They also require more advance planning; Swiss domestic trains run at least once an hour on all routes.
As for things to do, both Scandinavia and Switzerland are great, so you'll just have decide which one you're more in the mood for. Have you all watched Rick's various videos of these destinations? I find that's a great way to break a tie: https://www.ricksteves.com/watch-read-listen/video/tv-show
If you want to hear more about these places in person, come to our next New York RIck Steves Travel Meeting, on Saturday July 15 at 1 PM at Panera Bread, 425 Fifth Avenue (between 39th and 40th Streets).