I'd really try for both! Tornado at Bakken is definitely worth the trip, there is a part on it that is INSANE! I have been to both parks and I'm thinking if you are a coaster guy, I'd head to Bakken. BUT Tivoli can't be missed either!
Also is the new ride open at the park??
I agree with Chris, which is rare. If you're looking for more of a coaster experience, then I think Bakken is just a little bit better. Tivoli is one of my favorite parks in the world, but in terms of coasters it's somewhat lacking.
The more I think about it, you could do both in a day. There's a train station about a mile away from Bakken, and it runs directly to Tivoli (Central Station). Just make sure you hold on to your ticket, right Chris? Start at Bakken in the morning, then head to Tivoli in the late afternoon. Seeing it at night is well worth it.