Either works if both will genuinely develop and direct the business. The practical difference is small because an E-2 spouse is employment-authorized incident to status and may work in the dealership anyway; two principal applications mean two full investor cases to document.
One principal and one working spouse is usually enough
The E-2 spouse is admitted in derivative status and, under current policy, is treated as employment-authorized incident to that status, which permits her to work in the dealership, manage its parts counter or run the books without a separate application. That means the household loses nothing by naming one principal investor. Two principal applications are sensible when both spouses hold ownership in their own names and each will direct the enterprise, because it removes any dependence of one spouse's status on the other's, for instance after a separation or a death.
It also doubles the consular preparation. Children under 21 may accompany as derivatives and study, but may not work in the shop; a child who turns 21 needs a route of his own, and there is no E-2 path to permanent residence for any of them. If an adult child already works in the family's Alberta operation and expects to join the U.S.
dealership, plan a separate category for him rather than assuming the family visa covers him.