“The level of personalization and the attention that I get from S.P.G. is a huge comfort,” said Ryan Huff, a consultant from New York who has averaged over 100 nights a year in Starwood hotels since 2008. “I send my ambassador a Christmas gift and know about her family, and she knows my wife.”

“They get to know all your likes and dislikes,” said Charley Cullen Walters, an Ambassador-level member from West Hollywood, Calif., who runs the firm CW3 Public Relations. Mr. Walters happens to like chocolate-raspberry mousse, and most Starwood hotels know to greet him with that dessert upon check-in. But it’s not all about the frills; his ambassador has helped with complex booking needs for conferences and clients, and he’s worried that service will deteriorate as Starwood gets folded into the much larger Marriott network.

Starwood customers are right to worry, says Gary Leff, who writes the View From the Wing blog, about travel reward programs, because the two companies have fundamentally different business models. In the title of his blog post about the merger, he declared it’s “time to start gnashing teeth.”

“In general, the smaller programs are ones that treat elites better,” said Mr. Leff, who maintains upper-tier elite status with Starwood, Hyatt and Hilton. “It’s not hard to just fall out of an airplane into a Marriott. You have to make a choice to be loyal to Starwood.”

Starwood has nearly 1,300 hotels worldwide, with an emphasis on luxurious and distinctive hotels under brands like W, Le Méridien and St. Regis. Marriott has over 4,200 hotels, with particular strength in the kinds of properties that don’t offer a lot of frills. Because Marriott can count on its geographic reach as a selling point — no matter where your sales meeting is, there’s probably a perfectly adequate Courtyard by Marriott nearby — it does not need to be as aggressive about personal service.