The longest walk between gates at MEM is probably shorter than the average walk at MSP. Your OKC flight will probably come into a C gate at MSP and go out of a G gate, which can be a hike. The evening flights into MSP are risky, usually because the inbound to your origin often runs late. You won't have much time to use the lounge in MSP, and you'd probably have to use the F/G lounge anyway, which is reminiscent of a high school cafeteria. The MEM lounge is nicer but can get crowded. And there is always the BBQ in MEM.

Go with MEM because of the above and the longer connection. Much less risky. Your perfect 5C seat won't do you any good if it goes out empty while you're enroute from OKC.