This is not a rational answer to the question of whether it is more ethical to sign up for cryonics or donate organs, it is a rationalization. If magfrump decides it is more ethical to sign up for cryonics than donate his organs, he must decide this based on the ethics of those two choices. Someone might rationalize that it’s OK to be less ethical ‘over here’ if they’re more ethical ‘over there’, but it still doesn’t change the ethics of those two choices.
The exception is if the ethics ‘over here’ and the ethics ‘over there’ are inter-dependent. So donating money to a family in a poor country would be ethically relevant if one choice facilitates donating to the family and one does not. Here, we have the exact opposite of what was suggested: magrump can use the money he saves from not signing up for cryonics to help the family, and so he can consider this an argument in favor of the ethicality of not signing up for cryonics.
(But, magfrump, I would add that we also have an ethical obligation to value our own lives. The symmetry in ethics-space can usually be found. Here, I can identify it in the hypothetical space where cryonics works: the person who needs an organ can also be cryonically suspended, perhaps until an organ is available. Then you both could live. In the space where cryonics doesn’t work, organ donation is more ethical, since at least one of you can live.)
This is not a rational answer to the question of whether it is more ethical to sign up for cryonics or donate organs, it is a rationalization. If magfrump decides it is more ethical to sign up for cryonics than donate his organs, he must decide this based on the ethics of those two choices. Someone might rationalize that it’s OK to be less ethical ‘over here’ if they’re more ethical ‘over there’, but it still doesn’t change the ethics of those two choices.
The exception is if the ethics ‘over here’ and the ethics ‘over there’ are inter-dependent. So donating money to a family in a poor country would be ethically relevant if one choice facilitates donating to the family and one does not. Here, we have the exact opposite of what was suggested: magrump can use the money he saves from not signing up for cryonics to help the family, and so he can consider this an argument in favor of the ethicality of not signing up for cryonics.
(But, magfrump, I would add that we also have an ethical obligation to value our own lives. The symmetry in ethics-space can usually be found. Here, I can identify it in the hypothetical space where cryonics works: the person who needs an organ can also be cryonically suspended, perhaps until an organ is available. Then you both could live. In the space where cryonics doesn’t work, organ donation is more ethical, since at least one of you can live.)