They do the same thing.
After you've used "bzr merge-into ../other" 1 time, you can continue to use "bzr merge ../other" from then on to pull in any new changes, and it will find the existing files and update them.
They do the same thing.
After you've used "bzr merge-into ../other" 1 time, you can continue to use "bzr merge ../other" from then on to pull in any new changes, and it will find the existing files and update them.