Is it harder or easier to be an aviation mechanic or a car mechanic? Do you have to go to school for both of them? How long?

Except for working on my own cars, I have not been a car mechanic. I can't tell you if that is easy or not. I am an airplane mechanic (Airframe & Powerplant Mechanic (A&P)) and a Mechanical and Aerospace Engineer. You ask which one, car or airplane mechanic, is easier or harder. Being an A&P mechanic is easy in many ways. Since you cannot stop the airplane on the side of the road when you are in mid air and your engine stops or something breaks, the airplane needs reliable parts and a good maintenance plan. This means that as a mechanic you usually swap parts on a schedule and don't tinker with repairing a removed part (many can be sent out for overhaul by a licensed technician). You keep things clean and organized. Troubleshooting varies greatly between airplanes. The most modern airplanes tend to 'tell' you what is wrong with them and you follow the steps in the manual.

You can work on the body of the airplane or helicopter (airframe), riveting, fixing composites, etc. Or on any other components like fuel systems, oxygen systems, electronics, etc.

There are various ways to getting your A&P license. You can take the 2 year program at a school (see http://www.everettcc.edu/) and then take the FAA exams. Some work as aircraft mechanics (many in the military) and get 'credit' with the FAA for their hours, then they can take a shorter course and the exams.

The regulations to become an "airman" are in Title 14 Part 65 of the Code of Federal Regulations: http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/MainFrame?OpenFrameset
Click "by Part" and open Part 65. All the aviation regulations are in this Title 14.
Best wishes!

Picking a career should not be about what is easier or harder. Ask yourself what your passion is and what you would enjoy more. Most of us work because we have to, so it may as well be something you enjoy doing. Very few of us will say "Thank God its Monday!". I enjoy working on cars as a hobby, but I absolutely love working on aircraft. Both, cars and airplanes, will present their individual challenges. Remember that aircraft will have a lot more systems and is a more complex machinery, that will require a lot more knowledge. There is also a lot more responsibility, when flying 600MPH at 50,000 ft., there is no margin for errors. The decisions you make in aircraft maintenance, could affect the lives of those onboard, therefore a great degree of dedication and professionalism is required. Aircraft maintenance could also be thankless job, ever been in an airliner when everyone claps that the pilot landed the plane, but no one clapped for the mechanic that worked all night to make sure that everything works as its supposed to?
If, after all that, you still choose aviation, you need to figure out which type of aviation and what aspect you would like to be involved with. There is military, airline, and general aviation. Each one has its own rules and ways of getting there. Once that decision is out of the way, ask if you want to work airframe, powerplant, avionics/electronics, structures, nondestructive testing, accessories, etc, etc, etc. You don't have to specialize in one thing, but it helps in advancement and pay, and you get to do what you enjoy most. Also, remember that most aircraft are flown due to a business need, which means that a lot of maintenance happens at night or on weekends when the aircraft is not in use. Sometime the aircraft might have to be worked outside in inclement weather.
Aircraft maintenance should be your decision because you eat, breathe, sleep airplanes and can't think of anything else you would rather do. Its not easy, it takes a great deal of work and dedication, but it is immensely satisfying and rewarding.

I think you are not asking the right question. What do you love more? Do you like Turbines or Internal Combustion Engines? When you make your mind then you can plan your journey towards your dream. My journey took me over 10 years and it was worth it!

If you go with aviation, this might help: http://www.aviationmaintenance.edu/programs/
If you go with cars, this might help: http://www.lincolnedu.com/schools/lincoln-technical-institute