Copy or rsync command

Rsync is better since it will only copy only the updated parts of the updated file, instead of the whole file. It also uses compression and encryption if you want. Check out this tutorial.

rsync is not necessarily more efficient, due to the more detailed inventory of files and blocks it performs. The algorithm is fantastic at what it does, but you need to understand your problem to know if it is really going to be the best choice.

On a very large file system (say many thousands or millions of files) where files tend to be added but not updated, "cp -u" will likely be more efficient. cp makes the decision to copy solely on metadata and can simply get to the business of copying.

Note that you might want some buffering, e.g. by using tar rather than straight cp, depending on the size of the files, network performance, other disk activity, etc. I find the following idea very useful:

tar cf - . | tar xCf directory -

Metadata itself may actually become a significant overhead on very large (cluster) file systems, but rsync and cp will share this problem.

rsync seems to frequently be the preferred tool (and in general purpose applications is my usual default choice), but there are probably many people who blindly use rsync without thinking it through.