This decision will ultimately come down to which one will best help you tell your story. If you have two main things you want to discuss in your cover letter, the narrative format will enable you to provide more details about these items. It will also be the best way to use the available space since you don’t want to leave a noticeable amount of blank space in your letter.
If you have three or four items you want to discuss in your letter, using the bullet format may be the more suitable choice for you. This will allow you to make shorter notes without having to worry about transitioning between sentences, which can eat up valuable space. The choice between formats is based on your preference and which one you believe will help you produce a convincing cover letter.