"당신을 예뻐요" and "당신은 예뻐요", are both correct?
If they are, what's the difference?

Question

Answer

"당신은 예뻐요" is correct。"예쁘다"is an adjective
"ㄹ/을 " + Verb

Answer

당신은 예뻐요. = You are pretty. (Correct)
당신을 예뻐요. (Wrong) 
because 
당신은 -> subject 
당신을 -> object 
so '당신을 예뻐요' needs the subject
we can say 
나는 당신을 사랑합니다. = I love you