Monoalphabetic cipher:

1. The monoalphabetic cipher uses a single alphabetic key to map the plain text letters to ciphertext letters, so the relation between plaintext and ciphertext is one-to-one.
2. It is very less secure and susceptible to brute force attacks.
3. The space between the letters is left blank.
4. It holds the frequency of letters is the same as in the original message.

Polyalphabetic cipher:

1. polyalphabetic cipher uses multiple substitution alphabets to map the plain text to ciphertext. so the relation between plain text and ciphertext is one-to-many.
2. Relatively more secure, brute force doesn't work.
3. Here, space between the letters are filled with some other letters
4. It doesn't reveal the frequency of letters in the original message.