With Citi's program (2-4-1 domestic ticket), you CAN choose the airline if it is available. You just need to ask. Most likely though, if you don't book far in advance, the airline of your choice will not be available. I don't think the Citi program extends outside of the United States, but I may be wrong.
With American Express (2-4-1 full-fare international ticket), you can choose the airline, but there are limited airlines in the program that may not be serve your city directly. For example, if you plan on flying to China and in the midwest, most of the airlines don't serve that area.
Most importantly, you DO NOT need Citi's Chairman card to have access to the 2-4-1 domestic ticket program. It is available to the Citi's Premier Pass Elite. I would have to agree that the Chairman Card is no competition to American Express Centurion. While the cards may share the same color, the Centurion Card offers far more travel benefits.