Should I buy the used 525i or 530i..?

A friend of mine currently deciding which model to purchase BMW 525i or the 530i. He said that he test drove the 530i and leaning more toward purchasing the 530i. And he asked me specific question:
"From an economic standpoint for an additional $4k for 35 additional horses-- it is worth it?? Do you wish you would have paid extra for the 530i to gain the additional horsepower??"

So i will answer it from both functional and economic point of view in my blog.