(Corrects story link in paragraph 5) June 25 (Reuters) - Proxy advisory firm Institutional Shareholder Services Inc (ISS) has urged shareholders of Spirit Airlines Inc to vote for a proposed merger with Frontier Group Holdings Inc. "On balance, support for the merger with Frontier on the revised terms is warranted," the proxy advisory firm said in a report published late Friday and made public on Saturday. Spirit is the subject of a bidding war between Frontier and JetBlue Airways Corp. The proxy advisory firm last month asked Spirit shareholders to reject Frontier's offer, saying JetBlue's competing offer of $30 a share is superior from a financial standpoint. Earlier this month, New York-based JetBlue sweetened its offer for Spirit by $2 to $33.50 per share in cash.