If there is a manager who is better than your manager, and he asks you more than one time to join him. However, your current manager says, I can't let you go because of your good work. What would you do, and what to choose?

your comparison must be in related to the offer, that is really matters rather than managers. The offer is better than your current work then you have the right to convince the current manager to let you go. your career development is more important than current commitments.. In a diplomatic way you have to convince your current manager that make him either increase your current benefit or let you go.. 

i would say both of them give me an offer i cant refuse. (considering both of them are impressed with my work and abilities) 

I will stay and complete the project before leaving or joining to another manager,I have my own reputation as a proffessional and I have to keep it.Proud and happy to complete the project!!!

The current manager should give you an offer that you cannot refuse ;)