It's all about context. I tend to write a function in a fashion such that similar code looks similar. For example, if a function starts with a bunch of checks and one doesn't "look right" with a single line ... if (...);, then I use if/unless (...) { ... } for all of them.

Another important piece of context is the importance of the piece of code. If the line is important, then I might choose if/unless (...) { ... } to make more visually impressive.