Military Relationships! and Branch Question?

So I have been with my current boyfriend for 4 years now. We have had a long distance relationship which has worked out well. He has been speaking to me lately about joining the military. Now, I don't think there is anything wrong with the military and in my opinion it is a very honorable position for a man to hold. However I can't help but get nervous when I imagine him gone. Alot of people tell me that this will be easy for me because we have held a LDR for a while now. I think what worries me most is not knowing where he is and whether he is safe or not. All in all I'm worried something will happen to him. I know I'm probably naive about the way the military works and thats why I am on here. I want to hear from other military gf's, and ask how they've dealt with it, and what I can expect. I can't talk him out of this and I already know that but I want to be supportive and to be able to do that I need to know what I am going to be dealing with. Also, what is the difference between the Marines, Army and Air Force? I know he is leaning toward Marines but I've heard Air Force is easier to deal with. I'm not sure. Thanks.