唯一区别是情感色彩不同。

If I were you更加正式，适用于书面语或者正式场合下；

例句：If I were you, Mrs Gretchen, I just wouldn't worry about it 要是我是你，格蕾琴夫人，我一点都不会担心。

if I was you比较口语化，适用于和朋友聊天的场合下。例句：If I was you, I'd watch out who I was talking to. 如果我是你，我会提防那个人。