Will following a famous professor as a research internship be bad?
This is indeed a tough choice. 

In principle, if you manage to establish a strong positive connection with the "famous professor" and will collaborate or simply keep in touch with him, this could be beneficial for the rest of your academic carrier: he may not only write a strong recommendation letter for grad school, but also introduce you to his professional network (traditionally conservative academic circles will accept you more easily), help to find a postdoc and a tenure-track position, and, finally he could write a letter for tenure promotion for you. On top of this, if you will continue to collaborate with him, your co-authored papers will be more easily accepted by good journals, etc.

On the other hand, "as they said mostly I will follow his phd students." This is a very strong indicator that he will not have time for you (this is typical for famous professors with large groups) and you will be supervised by his PhD students. In this case, interaction with the professor will be minimal, and, if he agrees to write a letter, he will ask students who supervised you to assist. There is a chance that you would remain a sort of unknown to him. 

The "less-famous" professor, however, is likely to have more time for your. And you could learn (and benefit in other ways) much more from him than from PhD students of the famous professor. Remember also that a very strong and enthusiastic letter from a less-famous professor is better than a modest letter from the top one. 

So, as I said, it is a tough choice. 

I would recommend:


Try to find out what happened with other undergrads who where interns in both  groups (famous and less-famous). Where are they? If possible, contact them and ask about their experience.
Focus on how comfortable it is for you to communicate with both professors on a personal level. You could also ask them how often you should expect to interact with them. 
Don't reflect too much on "The thing I am concerning is as he is a leading person in top universities, he must be surrounded by lots of smart persons." You got the internship. They would not offer it to a stupid person, since they are "surrounded by lots of smart persons". You are smart.