you mean “Restore From Backup” vs. “Add Second Device”?
from the result, both options are the same and result in a completely independent device (in contrast to many other messengers that require the first device to exist)
the difference is the convenience - in most cases “Add Second Device” is easier - “Restore from Backup” should be done only when the first is not possible for whatever reason.
see FAQ - Delta Chat for further details.