I think if you're really concerned about efficiency, you should write to a binary file.
I'm not sure you've covered all the possibilities here. One reason it's efficient to write a large block at the same time is that you force the operating system to look for a location on the disk where it can put all the data. If you write one small chunk at a time, the operating system needs to find space for that data on each write, which can be inefficient and leads to fragmented files. However, if you know roughly how large your file will be (making it slightly too large is fine, you can shrink it later), then you can preallocate that space by setting the file size before you start writing. In this case, writing in the loop may be more efficient, especially if you would be building the array inefficiently. It also allows you to write one chunk while you process the next (the operating system will normally handle this for you by queueing the disk operations; the actual writes to the disk are asynchronous to your code).
