两种用法，意思差不多。但后者很少见，远不如前者普通。

1. ability to do sth：不定式作定语，表明不定式动作的“情态意义”。其特征是，可以使用其同源形容词，转换为：be + adj. to do 结构。

her ability to make people feel at ease =

She is able to make people feel at ease.

2. ability in （doing）sth：介词短语作定语，“在…方面的能力”。例如：

Good. Tell me something about your ability in written English.  

很好。请告诉我你在英文写作方面的能力。

来自轻松英语会话---联想2000词

Besides, doing translation exercise may also help you improve your ability in using the language, thus making preparation for improving your writing ability. 

另外，适当做一些翻译练习也可以帮助你提高运用语言的能力，从而为提高写作能力做好准备。