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It *would* be safer, in many ways. If you launch your antimatter rocket out in the middle of nowhere, like, say, off a floating platform in the ocean, if things go wrong theres a flash and that's the end of it. If the payload is a nuclear reactor, though, you have at least the potential for the uranium to be vaporized and create uranium oxide hazards downwind.

I don't know if the astronauts would consider it safer! 10 milligrams is equivalent to 500 tons of TNT. And if that's not bad enough, the exposure from the gammas would unquestionably be fatal. At least with a chemical rocket you theoretically have a chance to get away from a dangerous rocket with an escape mechanism (like on the Apollo program).

I think you overestimate the danger with a nuclear reactor. Nuclear reactors don't suddenly explode and vaporize everything (especially their fuel). And even if they did, you could apply your caveat of being in the middle of the ocean. Any way you look at it (unless you have accumulated significant radioactive material in the reactor core), the nuclear option is safer. The nuclear option will only be dangerous if you take active measures to make it that way (violate safeguards, etc). The antimatter option is only *safe* if you take active measures to make it that way (ensure confinement).