I'd press the "PRESS THIS BUTTON" button.
Why? Because I'm not an idiot who can't read or follow simple instructions.
I'm aware that it could be a bluff and each button could have an equal chance of being the wrong button (or both buttons could do the same thing), but if I am going to meet my end in such a situation, then I'd rather not meet my end looking like an idiot who pressed a button that clearly stated "DO NOT PRESS THIS BUTTON".
In the worse case scenario where I can't avoid my fate, at least I can have some control over how dignified the circumstances of my fate appear.
Also, if the buttons aren't for me and instead one of them opens a trapdoor which drops another person and their pet into a whirlpool, then at least if I caused this by pressing the "PRESS THIS BUTTON" button, I have the defence of coercion and can possibly get away with manslaughter, which is better than causing someone's death by wilful ignorance of a simple warning or an active decision to act contrary to instructions.
At the end of the day, in it's most basic form and without any other information to the contrary, pressing the "PRESS THIS BUTTON" button is doing the right thing, whereas pressing the "DO NOT PRESS THIS BUTTON" button is doing the wrong thing, so whatever the consequences of pressing a button are, I'd much rather cause them by trying to do the right thing than by consciously doing the wrong thing.