Since the visa cost will be the same, do ask for what you--as a US passport holder--are entitled to by treaty, which is a 10 year, multiple entry tourist visa good for 60 days each stay. On the application form section 2.2 for "intended number of entries" check the "Other" box and type in "Multiple entry valid for 10 years" and section 2.5 "Longest intended stay" put "60." Most reputable agents will try to get this for their clients anyway, but it helps to fill out the form in parallel. As long as your current passport has at least 12 months of validity, you should get this visa and it will carry over into renewal passports. Consider it an investment, in case you want to do future visit or transits or mileage runs to China, you're covered for the next 10 years.

Make sure the agent knows that you want a tourist L visa, even though the purpose of your first visit is Transit.