OK. Here's my take. First, if you plan on marrying your girlfriend, she must, absolutely, 100%, without reservation, be on board with your joining up. I will be honest here. Military service WILL put a lot of strain on a relationship. If your relationship is strong, you will likely get through things OK. I won't pretend to know anything about your relationship status, but I will say that I have seen TONS of marriages fall apart after one party or the other has joined the service. Please don't take this as me trying to discourage you, but you should both be aware that there will be lots of seperation and uncertainty, and that can cause problems for the strongest of couples.
If you plan on making a career of the military, being an officer is the way to go. If you're planning on a minimum service time obligation, enlisting is a great option too. You can have a shorter active duty service time commitment, and there are lots of more interesting and fun jobs available to an enlisted man. If you go in as an O1, you will be doing more admin crap than you will ever care to do. I always felt sorry for the 2nd lts (and some 1st lts) that came around. Taking crap from every direction for their lack of experience. I wouldn't think that you would have a very fulfilling term of service if you went minimum service length as an officer. By the time you have a chance to develop some respect and leadership ability, you would be looking at leaving. It just seems like a waste to me to lay all the groundwork, and complete all the schooling to become an officer to just walk away from it once you have completed the minimum required time. If you want the "adventure" of joining the Army, or being a naval security gaurd
, I would enlist and go into one of the combat arms branches. Like infantry, cav, or something similar. Try to get jump school in your contract, or even Ranger training. Sign up for the GI Bill to help pay for grad school when you get done.
As far as being put at a disadvantage upon completion of your grad school or law school, don't fret that at all. Your peers from your your class will be at a disadvantage due to your superior resume with military experience on top of your school accomplishments. I know who I would hire if I was presented two resumes with equal school credentials, but one with military service to go with the education, and one without. Your career down the road will only suffer in that you will be getting a bit of a late start. The experience and leadership ability you will gain will only help you in the job market when you get out.
I say go for it if that's what you want to do, but make sure you go into it prepared, and make sure you think very thouroughly about what you want out of your experience. Also include your girlfriend in all discussions and in the decision making process if you really are that serious about marrying her.
Good luck, and thanks for considering serving our great country!
PM me if I can help in any way with your decision.