Re: What is your art?
It's Katana, not Kitana. BJJ is inspired from Jujutsu, which is the samurai art. You really gotta polish up your basic knowledge man...
Either way, they aren't really similar, but they help each other in that they're both artforms that involves manipulating the human body, and gaining control of the other person. It's nice to be able to fight on the ground well as a budoka, and certainly BJJ isn't great while standing. In this way, you become more well rounded in fighting, and so gain a superiority in terms of pure combat prowess.
This is essentially true mixed martial arts, not the cagefighting that's all the rage now. Throwing away not just all rules, but also all politeness and respect, you must consider fighting in all forms, whether you are striking with long reach and far kicks, up close with elbows and knees, grappling and throwing, or even pinning and wrestling. Not all martial arts have all of these combined, so one answer to this problem is cross training.
Aikido came from Daito-ryu Aikijujutsu, which came from the Takeda family jujutsu style. This is the original samurai art form, the jujutsu of the 18 arts of the samurai (kenjutsu, kyujutsu, bojutsu, etc.). In this case, samurai bushi's never had to fight on the ground, as once you fall in full armour, it's really difficult to get up again, and most fights aren't one on one, but in armies. If you fall with your opponent, your opponent's buddy will just stab you and help his friend up.
BJJ is meant for one on one. They don't really consider what happens when you get attacked by another person while pinning the first one, so that's one major weakness. But should an Aikidoka falls to the ground, he would certainly love to have BJJ backing him up.
I think it's best to gain a striking style like Bajiquan, Wing Chun, Karate, or Muay Thai (Muay Boran if you can get that, woo!), plus a wrestling form like BJJ or even Pankration (ancient styles FTW!), in addition to Aikido, I think would make a very complete and deadly MMA fighter. I'd love to see an Aikido based MMA fighter in the cage matches, just to shut the nay-sayers up. The old masters fought all challengers, why shouldn't we?
But I guess what I'm saying is there's similarity in that we're learning a fighting art, and that'll make you stronger regardless of how it fights.