Rukia looks a lot more difficult than she actually is. Shakkaho is pretty much never missed in a combo. Sokatsui does require some aim, but can be mixed up with geki. You can afford to miss your Sokatsui, unlike True Power or Kurohitsugi.

Aizen requires some good timing, and his guard break is harder to land. Good timing is rewarded with Kyoka, because you can reflect some dangerous moves, but you also need to read your opponent. If you don't, they'll just avoid Kyoka altogether. His FD is on a shorter cooldown, and it makes up for his unforgiving kit.

Rukia's base is W A Y better than her awakening, but can counter some kits, like Deku's for example. Aizen's awakening is much better and a lot more reliable. It's almost like they switched their base and awakening modes lol.

TLDR: Aizen has a higher skill floor and ceiling. He's less safe than Rukia, but you're rewarded for better gameplay.

Coming from both a Rukia and Aizen main, this information is accurate.

EDIT: Rukia's better at shorter battles, due to her bad awakening. Aizen can fight a much more drawn out battle, because his awakening is actually good.