I want to be a little skeptical. First group of people may live a happy life if they want to. By helping others and needy people. By giving Charity and Zakat (applicable for rich Muslims), they can purify their wealth and soul. In such way, despite of having huge money, one can be happy.

On the other hand, if a poor person always want to be a rich one or want to compete with others for worldly gain, he may lead an unhappy life.

Those who are greedy and only want money, money and money, they can never be happy, whether rich and poor,until they meet their death.

Allah says in Quran:

Competition in [worldly] increase diverts you,
Until you visit the graveyards.
At-Takathur 102:1, 2