I believe an item that has a half-life would decay at the same rate regardless of temperature (maybe I am wrong on this assumption) but it would definitely slow down due to time dilation above an event horizon. Are there other differences?
Event horizons represent regions of strong spacetime curvature and gravitational potential. Hence they affect the flow of time in characteristic ways. Clocks (including radioactive decay) run slower as seen by infinity, signals from them are redshifted, there is Hawking radiation, there are big tidal forces if the central mass is small, etc. These effects are non-local in nature: they happen because of the overall field, not just the conditions at the item itself.
Absolute zero temperature is a property of the item. It is when the item is in its ground state with no extra excitations. But that still allows radioactive decay or acting as a clock by rotating. There is no time dilation, no redshifting, no Hawking radiation and no tidal forces. The only thing in common is that an observer will likely find the item to be slowly changing.