A letter to compliment your service is far more appropriate and very gratefully received by both the company and the cabin crew.
Is this really the case? Does an airline really expect to be bombarded with complimentary letters, assuming their front line staff are doing their job well? I am not an irregular traveler, I like to think I am a courteous and considerate one but I have never had cause to write such a letter. Thank you(s) of course, but letters!!