"Their names were in the tenth letter."
Translation:Jejich jména byla v desátém dopisu.
It's a matter of personal preference. I wouldn't say one ending is used more frequently than the other. It's the same as "autobusu/autobuse" and many other "hrad"-paradigm words in the locative singular. Some people use one of the two endings consistently, others alternate between them.
In case you want to read a more thorough analysis: http://prirucka.ujc.cas.cz/?slovo=dopis#bref4