Wouldn't make any sense for them to join the SEC. Geographically....the Big Ten makes far more sense....including two in-state Big Ten teams. I don't think they have any rivalries with any SEC teams....while they do with USC, Purdue, Michigan, MSU, and Northwestern. They also have a serious hockey program and already compete in the Big Ten hockey conference.
Basically....once ND decides to join a conference....I'd say there's little chance they go to the SEC over the Big Ten.