Choosing between methods and functions is never a matter of reusability, and quite rarely a matter of speed (anyway, hundreds of times per minute means several times a second, which is almost nothing).
The main points to consider are
- Will you prefer to think in functions doing something to objects or in objects doing something at your command?
- Are you going to inherit from the object and override its behaviour?
In case of reading file line by line I would suggest a simpler approach—unless of course you're doing an OOP assignment or doing OOP just for practice. It's not that as if there were anything wrong with OOP—it's a great technique, by all means use it when you need to model complex behaviour. It's just that for a simple script it's almost always an overkill.