My wife was at Mizzou during the Woody's Wagon years, when OU and Nebraska would routinely beat the tar out of Missouri in football.
I don't think there is much love lost for those "rivalries". Maybe Kansas, though.... that dates back to the Big 8 (100 years in football).
We both think Missouri might be best suited to be in the Big Ten. They seemed to have abudding rivalry with Illinois.
Certainly being in a stable SEC beats the hell out of being in a Big XII where half the teams care nothing about leaving he other half high and dry.
SEC will be (as they say) a challenge and an opportuinty.
It should help recruiting, and if Mizzou plays their cards right, they could end up elevating their national profile.
On the other hand, the competition level (especially in football, swimming, and a few other sports) is intense.
Keeping this somewhat on topic...The changes in the NAIA recently have been rather alaming. NCAA D-II is still the smallest division of all the "College Associations", but if the NAIA keeps losing schools at the rate it is, it will be dead in less than 30 years.
As for the Big 10 thing, the majority of Mizzou folks that I know basically said to hell with that league after being "turned down" for the "third" time (After the 60's when Devine was the AD, in the early 90's when the Big 8 was turning into the Big 12, and after NU being taken instead of us). Most people here realize the challenge coming on, but also realize that this may be the best overall fit after this most recent round of reshuffling.