ZIP format uses a proprietary encryption algorithm.

RAR archives are encrypted by the much stronger AES-128
standard. If you need to encrypt important information,
it is better to select the RAR archive format.

ZIP:

The Zip 2.0 encryption format is known to be relatively weak,
and cannot be expected to provide protection from individuals with
access to specialized password recovery tools.
If you have important security requirements for your data,
you should look into more robust cryptography solutions.

RAR:

ZIP format uses a proprietary encryption algorithm.
RAR archives are encrypted by the much stronger AES-128 standard.
If you need to encrypt important information, it is better to
select the RAR archive format.
For real security use passwords of at least 8 characters in length.
Do not use words of any language as passwords.
It is better to choose a random combination of
characters and digits.
Note that passwords are case sensitive.
Remember that if you lose your password,
you will unable to retrieve the encrypted files.
Not even the WinRAR author is able to extract encrypted files.
(see the chapter Encryption in the WinRAR help file)