If you can get away with using export or save, do so. The overhead for a custom reader/writer is rarely worth the small amount of storage space you'll save. However, if you are writing thousands of lines, and you are concerned about read or write times, then it is best to write a slow reader/writer, so that it doesn't read/write the entire file at once, but rather schedules it out so that it only does a few hundred lines at a time.
Writing a saving/loading system can be quite fun though, and its a good learning experience.