A unilateral notice is not very different from an agreed notice, since they both serve the same purpose. However, for this one, you will not need the proprietor's consent, nor will you have to present supporting evidence for your claim. In addition, if you choose this type of notice you may benefit from more confidentiality, since the wording for it in the register is more limited.
There are certain interests that a unilateral notice cannot cover, which is why an agreed notice land registry might be better suited. You will also need to fill in a different form when requesting its cancellation.