I realize that Auburn has a higher ceiling than Ole Miss, and they can spend more on facilities, but there are other issues for Kiffin to consider. Auburn is not an easy place to coach. You're always going to be second fiddle to Bama. The booster interference and warring booster factions makes for a difficult life there. No matter what you do, half the boosters are actively working against you so that they can put their guy in the corner office. The fanbase is maybe the most cultish in all of CFB and there's an attitude in the fanbase and program that's not conducive to sustained success. Note that Auburn has won 10 or more games in consecutive seasons only once in their entire history... and that was way back in the 80's. Even with seasons now having 12-15 games, they still can't sustain any sort of momentum or success. They consistently take short term approaches to long term problems.
Kiffin could get a raise and better facilities, and have a better shot at an SEC crown and the playoffs, but odds are it would eventually become a career ending job, as it has been for others. Auburn is a frustrating, difficult, high risk job. Lane can stay at Ole Miss for awhile with an easy job and be an incredibly appreciated hero there. A move to Auburn comes with enormous headaches and a likely disaster within a few years. Kiffin will get other better opportunities in the next year or two without nearly the problems and risks Auburn will entail.