Oradbo, my preference is I-90. I can give you some info about what you could see in So. Dak. Do you have plenty of time to see whatever you'd like or is there a bit of a time limit?
First of all, Mount Rushmore south of Rapid City is a must see for everyone. You'll be shocked at how majestic it is. Right down the hill from Rushmore is Keystone and you may want to stop there. It's the cutest little town. I believe you take 16A at Rapid and just head south. That is the one thing you won't want to miss while traveling through here.
If you aren't pressed for time, I'd stop at Sioux Falls when I get to So. Dak. Falls Park and their new Phillips to the Falls is really neat. There's a lot to see in Sx. Falls if you're interested. They have a travel guide, too.
If you liked the Laura Ingalls Wilders books or like "Little House on the Prairie", you could swing north and see the Ingall's homestead up at DeSmet.
The next stop I'd make would be to check out the Corn Palace in Mitchell. It's something nice to see if you've never seen it before.
You could stop at Wall and visit Wall Drug. There are signs for it all over the world. It was started years ago by the Hustead Family and they offered free ice water to travelers and it was a hit.
Murdo has a very nice museum with a lot of old cars and things. The last time we were there, they had Elvis' motorcycle.
I think you turn at Interior, but please rely on a map because I'm not good at directions.
This route will take you south so you can go through the Badlands. You'll feel like you're in another world.
The Black Hills region has so many things that you may want to ask specific questions on the So. Dak. forum. There's Mt. Rushmore, Crazy Horse, Custer State Park, the Cosmos, Deadwood, the Horse Sanctuary over at Hot Springs, Evans Plunge at Hot Springs, Reptile Gardens in Rapid City, the 1880 train, etc. There are also museums there, too.
I hope this will get you started, and if you have any questions, I will try to answer them for you and if I can't, I know just the person who can.