Personally, I'd rather sign up for cryonics. However, if your goal is to maximize the amount and quality of life lived, a plausible case can be made for either cryonics or organ donation. Organ donation will save some number of lives between 0 and maybe a dozen at best, depending on how you die. These lives will likely be elderly people who aren't signed up for cryonics. The money that would have gone to pay for your suspension can also be optimally donated to save some more lives; the most commonly tossed around number is 28 third-world lives vs. a high-quality suspension from Alcor. The benefit of cryonics depends on its chance of working, and on how long and happy your post-revival life would be. A detailed analysis is here. It came out that both options are pretty close, i.e. within the massive error bars of each other.

In conclusion, I'd say either preference is "okay." Go with your conscience.