Icandoit,
Why such a strong impulse to go directly to Law or Business school? I personally advise almost everyone I talk to about careers to go into the work world for a couple of years. You'll get the experience that you need to get into a more top-tier program and will be able to better fine tune what you want to do. You're only, what? 21? 22? It's silly to think that you would know exactly what you want to do now. I have many friends who went right from undergrad to Law School because they 'didn't know what to do'. They finished, not wanting to practice law, and with $200k in debt. A friend of mine who went to Yale Law School is now working in Psychology Clinical trials. Go figure.
Second, there's no MBA requirement for I Banking. Tons of my friends from school work at Goldman, Lehman, etc without an MBA. The typical path is to work 4 years and then return to get your MBA.
I'll offer the advice that my dad gives (he's an attorney) - don't get a law degree unless you want to practice law. You seem interested in corporate law and you've mentioned your enjoyment in going to court. In corporate law, chances will be slim that you'll hit the courtroom in the next 10 years.
Back to the question of MBA or JD, though, if even after my advice you still want to go back to school right away. I struggled with the same decision, too - and I have the 5 years of work. The benefit of a law degree is that you finish school and you're a lawyer. Immediately employable anywhere. You finish an MBA and you're just a better qualified business person. Slightly different twist, and much more open-ended. But with a JD, you can bet that even in the corporate world, you may be pigeon-holed into legal matters. They both can eventually lead to the same place - only JDs are typically in the background of deals, MBAs at the forefront. This is at the very top level. JD is 3 years, it's intense, and, from what my friends ahve told me, it's pretty miserable.
My final advice: go work. BOTH JD and MBA programs are beginning to value this more and more. Figure out what you REALLY want to do. If in 2 years, you still are considering both, look into combined JD/MBA programs. You're young and you should explore the working world. Hey, I agree that school is fun. I love learning. But the work world is not like school. Law school is not like being a lawyer, it's much more theoretical. Get practical experience.
Good luck!