It's hard to say who is a worse person, but here are some things to consider:
Angel leaves Tess and has no real intention of going back to her; he cheats on her with a stranger; he is unable to forgive her when she admits to her dalliance with Alec; he asks a milkmaid and not Tess to go to Brazil with him. However, once he returns from Brazil, it does seem that he has changed. Furthermore, there is an idylic quality to the first days Tess and Angel spend at the farm.
Alex is no saint either. He seduces Tess by taking advantage of her. He continues to follow Tess regardless of her marital status or her pleas for him to leave her alone. He is relentless in his pursuit of Tess and only under duress is he able to take her away.
From Tess' perspective, it would seem that Alex is the worse person-- after all, she kills him! however it's up to the reader to form their own opinion based on the evidence of the novel.