These two can confusing but they are actually to different things and very easy to understand
Independentely if the mail received on a mailbox is to be send internally (to another internal mailbox) or externally (to a mailbox from another domain) Mail Forward
is the correct choice if you want to send everything from the original mailbox to another mailbox
MAIL ALIAS is for instance if you have the mailbox firstname.lastname@example.org and create an alias named email@example.com or ConduRice@domain.com every message sent to either of the two
will end up in firstname.lastname@example.org
It's a matter of directioning, if you have a email@example.com and a firstname.lastname@example.org you will want to make a MAIL FORWARD from email@example.com to firstname.lastname@example.org, otherwise, if you use a MAIL ALIAS, any email sent to email@example.com will end up in firstname.lastname@example.org . Definitely not desirable.
So, in sum:
MAIL FORWARD from box1 to box2 when box2 is to receive contents of box1 but not the other way around.
MAIL ALIAS box1 with box2 when received mailed on either of the two will end up in both mailbox's