my opinion to BigMac,
take this with a grain of salt, as I've been a Marine a grand total of 1 1/2 years. My answer to you is like an answer someone gets when they ask the question, "How do you know she was the one to marry" The answer is, "You know when you know." I always thought that was a bs answer until I experienced it myself.
The same way with the military. My initial experience was with the air force, all my friends I knew in the military were air force and we had an air force ROTC at school. When I talked to the recruiter, he said you have to want to be an air force officer first, pilot second. I thought to myself, I just want to fly, so if you want to hear me say that crap, ok.
When I talked to the Marine recruiter, he said that you have to want to be a Marine first, pilot second. I actually believed this guy. He lived what he preached. At first my initial thoughts were exactly like my response to the air force recruiter, but after spending more time around Marines, I saw something different. I began to believe it myself. The reason I joined the Marine Corps is I wanted to be a Marine more than a pilot, and that was the first time that happened.
Now, the other services are good in there own right, you can just tell there is something different about the Marine Corps. You can call me idealistic, that I haven't been exposed to the bad parts of the Corps yet, and you'd probably be right. But, my advice to you is spend time with the services. See how they interact with each other and amongst themselves. Seek out people that have served and talk with them, find out their honest opinions. You'll soon get a clear picture of the cultures of each and which one you want to be a part of. Then you'll answer, "I knew when I knew."