It doesn't really matter. Both are excellent at protecting your assets and have nearly identical protections. Both will protect your assets immediately.
The main difference is that the Cook Islands will leave a 2 year window open for someone to "challenge" the transfer of your assets into the trust, before they shut that door completely. This challenge is not only a costly endeavor, but also practically insurmountable (the person suing you must physically travel to the Cook Islands to submit a claim, and also prove beyond any reasonable doubt, aka 97%+ certainty, that you moved assets in the trust for the express purpose of defrauding him specifically).
Belize? That door is shut from day one.