That's a good question. I live on the SE side of KC, so when I drive to Tulsa, I take US-71 down to Joplin, then I-44. To the Ponca City area, then it would be US-60 over through Bartlesville to Ponca.
I visit Bartlesville, OK once a month for work and I use my GPS for the best route. It takes me a different way every time. I punched up Mapquest and it took me down I-35, so I have to backtrack a little up in KC.
The short answer is probably I-35. Will you be going?
If you look at the first post in the thread it gives the time spent driving for each lap. The average speeds have been between 17 and 23 mph, I think they are using P&G between 15 and 45 mph. Note that the thread I hyperlinked is different from the one at the top of this thread.