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I'm trying to write some science fiction where they've had nuclear fusion working for quite sometime, interplanetary travel is no problem, and they're debating making the next step to a radical new, more efficient energy solution to run their space stations. What would that solution be? Antimatter reactors?

The answer is still fusion, but with different fuels. The easiest fusion reaction to do, and the one current research focuses on, is deuterium + tritium. But tritium has a 12.5 year half life and doesn't occur naturally. It has to be transmuted from lithium inside the power plant, right next to the reactor. This is a huge added complexity. The D+T fuel also emits high energy neutrons, which irradiate the surrounding materials and require shielding. You could pick a harder fusion reaction like hydrogen plus boron. Then the reactor itself has to be way higher tech, but things like fuel and shielding get a lot easier. There's also helium3 + deuterium to look into.

If you want to get really stupidly exotic, try to steal rotational energy from a rapidly spinning black hole, which is like the one thing you can take from it.

Fusion isn't likely to be a very efficient way to harvest electrical energy in the physics sense (efficiency = energy harvested per energy consumed) even today. It is so popular an idea because it is a promise of a cheap and limitless energy production, not an efficient production.

If what you are after is efficiency, hydropower plants are already close to 90% efficiency today, much better than fusion is hoped to be. If you can make the station so big that you have lots of water, gravity and water cycle powered by Sun in it, then you can put a hydroelectric power plant there.

Forget about antimatter as a fuel; the problems of producing it in quantity & storing it safely are insuperable. I suppose in science fiction anything is possible,but in the real world,much of science fiction fantasy is impossible & always will be. I'm afraid interstellar space travel falls under that heading too. As for energy to run the space station,all space stations are likely to be in the inner solar system,so what's wrong with solar energy? To have a fusion reactor on board a space station is likely to be very tricky,so why not have it on Mars or somewhere similar & beam the energy up to the space station by laser? On the other hand a nuclear fuelled (radio isotope) generator is quite feasible,& for safety reasons could be some distance away & connected to the station by cable.Then there is the question of avoiding space maladies like muscle & bone wastage,so why not insist that all astronauts go for a run on a treadmill every day & use the power generated? Or if you had a classical,wheel shaped space station,the running track could circle the perimeter & if all astronauts were made to run in the same direction,the space station would rotate in the opposite direction. This would create artificial gravity which would make life more normal for everyone.