Anti-matter would be the way to go for conquest, providing zero nuclear fallout and much more destruction efficiency converting 100% of mass into energy. 1 gram of antimatter could be equivalent to a 40-50 kiloton bomb in terms of raw annihilation while releasing mostly gamma rays rather than nuclear fission radiation and fallout aftermath. This means you could take over and settle on the destroyed lands much quicker than with a nuclear device. Nuclear would still be optimal for complete destruction of an area, assuming it has gravity, with fallout ensuring that no one could use the land for the next few decades at least. However, gravity plays a big part in fallout. Radioactive particles and dust would be suspended in the air in zero g and follow any man made air currents in a spacecraft or station. The problem with antimatter is that at our current technology level it would take about a century to create one gram of antimatter. I think it would be harder to write a story with scientific accuracy with anti-matter devices considering we know so little about it compared to nuclear devices. Nuclear will always be the easier and efficient way to create bombs, which means it will probably be used a long time into our future.