I am currently designing a financial app, which is more trendy and user-friendly, the use of lines or text boxes ?
Trendy is an underline.
User friendly is a box (bordered on all sides).
What might be trendy might not be user friendly. For instance, the hamburger used to be 'trendy', however it has known usability issues. This will also depend on your main audience. what age group are they? You could test both options on your audience to get their feedback.
When in question, I'd advise you to go for the safer and more obvious choice.