From man cp:
-R, -r, --recursive
copy directories recursively
-a, --archive
same as -dR --preserve=all
-v, --verbose
explain what is being done
-u, --update
copy only when the SOURCE file is newer than the destination file or when the destination file is missing
from man rsync:
-a, --archive archive mode; equals -rlptgoD (no -H,-A,-X)
-v, --verbose increase verbosity
-h, --human-readable output numbers in a human-readable format
-P same as --partial --progress
--progress show progress during transfer
--partial keep partially transferred files
The benefit from using cp instead of rsync for initial transfer is more faster processing of files (the cp command do not collect information regarding hashes of files and will transfer file as a whole piece, if it’s updated or doesn’t exist on destination), but you may mess it up, if you run it in the second and next times, because the cp -r may create another copy on destination instead of updating existing one, depending on how you provided the source and destination paths.
The last one
must be done anyway when the source node is stopped to remove wal and shm files for your databases, otherwise they may lead to databases corruption on start.