They are both speaking about how beautiful women are harsh and cruel.
The sonnet states, Soft as her beauty do behold, the writer is talking about how beautiful she is and speaking of the woman highly, and then you have the next line, And therewith do her cruelty compare.
Then you have the next sonnet, As those whose beauties proudly make them cruel; and then you have Thou art as tyrannous, so as thou art,