A poor guy basically is just saying that he lacks money. He's a wretch however would generally be used to describe a personality and means a worthless or horrible person.

He's a poor guy: he is poor; has little income or no money - there is no suggestion that his character is necessarily poor or lacking. He's a wretch - it can mean he is extremely poor, living in dire poverty of almost Biblical proportions; it could also mean he is of very poor character