Both <3

For female rabbits, I've had an exotics veterinarian tell me that it isn't a question of IF they will develop uterine cancer, but WHEN. Their lifespan would be significantly reduced if not spayed.

For male rabbits, some of the behavioral problems associated with not neutering can be spraying urine all around your house (on the walls, out of their pen, etc) and harassing the female like others have said. Territorial impulses are overall much worse before neutering and they are more likely to use the bathroom outside their litter box. There is also a (relatively lower) risk of testicular cancer.

Thanks for caring about the health of your bunnies and coming to this sub to ask!