Dino,
All you can be is honest with both professors. Don't feel guilt for one professor simply because you wish to work with another. Considering they are professional level of both, they should understand the time and commitment that goes into a PhD program. You will be spending the next couple years working together; you'll want to be sure your relationship will be sustainable long-term. They ought to be supportive of any decision you make.
If one of the professors becomes upset your decision, rather than showing support, then I personally wouldn't want to work that person anyhow.
In your post title you questioned if you had a moral obligation to anyone: you don't. You don't owe anyone anything. You've paid and worked had to be where you are; it wasn't free and effortless, and no one gave it to you out of the goodness of their hearts. Be compassionate and empathetic to those around you, but don't forget that the only that truly looks out for you is you.
Those are my thoughts for what they're worth. Good luck!