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 travel demand to falter. Many large investors take ISS's recommendations seriously when deciding how to vote on corporate proposals, director candidates and other matters. "On balance, a potential agreement with JetBlue would appear to offer shareholders superior optionality, allowing those concerned with the turbulence ahead to exit at a significant premium, while allowing those with a more optimistic outlook to reinvest," ISS said. JetBlue's cash offer represented a 56 percent premium to Frontier's cash-and-stock offer as of last Wednesday, ISS said.