Well, what's your background? What are your goals?
If it was me I'd do Judo hands down, but for all I know you've been doing Aikido for a long time and are really into it. The instruction for either one that you will get in Japan will be among the best in the world.
"Dominate" is a verb. "Dominant" is an adjective. A fighter can dominate his opponent. If he rules his division, he is considered dominant. Yes, this needed to be explained.