It's hard to define things in terms of better or worse. My sensei says to think of belts in terms of school. Everything till black belt is elementary school. Shodan is middle school, and nidan is high school. Thusly, the level of understanding is different.
As far as who can beat up who, a lot of that comes down to the individual, luck, and a lot of other things. As for rankings in other styles, I can't say. I've seen 15 year olds with 20th degree TKD blackbelts who couldn't punch their way out of a wet paper bag, and 1st degree karate blackbelts who were like machines.
I just try to worry about my training and let other people worry about theirs.