Advisory firm urges Spirit shareholders to vote against merger with Frontier.

Spirit Airlines’s shareholders should vote against a proposed merger with Frontier Airlines in favor of a competing offer from JetBlue Airways, a prominent shareholder advisory firm recommended on Tuesday.

The firm, Institutional Shareholder Services, said that while the rival offer from JetBlue might face more regulatory scrutiny, it would offer Spirit investors more money and more choice, depending on whether they expect the recovery in…