Hawaii remains the most popular choice of Americans for summer vacation travel, according to a survey released Thursday by American Express Co. The forecast, based on a mail survey of 350 American Express travel agencies nationwide, showed 27 percent of the agents polled named Hawaii as the top choice for vacation travel this summer. The other leading vacation destinations were Europe with 21 percent, and Florida and the Caribbean with 18 percent. Despite the fall of the dollar against major foreign currencies, the agents said most of their clients planning European vacations this summer are not changing their plans. Florida ranked as the leading family vacation spot, according to 72.5 percent of the agents surveyed, largely because of Walt Disney World in Orlando. Mexico is the summer destination growing fastest in popularity, according to the survey. It found some regional differences, however: Mexico is the fastest growing for travelers from the Northeast and the South, while the majority of agents in the Midwest named Europe as gaining quickest in popularity. For vacationers from the West Coast, the South Pacific _ including Australia, New Zealand and Tahiti _ is growing the fastest. Within the United States, the 10 most popular cities for vacations this summer, in order of preference, are: Honolulu, San Francisco, Los Angeles, New York, New Orleans, Washington, Miami, Denver, Boston and Chicago. The survey also found that price and quality of service are the key factors determining the choice of a vacation. Cruises are the most popular type of vacation planned for this summer, followed by all-inclusive resort vacations, escorted tour packages and weekend fly-drive packages, the survey showed. Single men most often choose all-inclusive resort packages for summer vacations, while cruises are the top choice of single women, according to the survey.