Do both.  Write to them, though.  Letters are often included in an employee's file and considered when the employee is being evaluated (raise?)  \n\nFirst paragraph should state that you are writing to praise two employees for excellent service and to make them aware of two others whose performance was less than glowing. \n\nLay down the events giving detail.  Phrases like "they were great" doesn't tell the company what they did that you thought was great.  Company's are just as interested in what people like enough to write about as they are to complain about.  This info goes to marketing.   Ever hear of the squeaky wheel getting the grease?  You will be the wheel getting greased next time changes come around.