I'm currently in the Army, you may want to take in to account that the funding per each branch is different. There is super pride with being in the Marine Corps I get it but there lack of funding and the equipment they use is shit. And those who argue with me about this can eat a dick. I'm not saying anyone branch is better than the other but if you want nice shit and want to work on nice shit (based upon what you said you want to do, Mech.) than I would suggest either AF or Army. Both have the highest funding per the governments wonderful budget.
I have been in for 2.5 years so I can tell you that you will meet and make the best friends of your life. You will travel alot and have fun. You can't say one branch is better than the other in any case. They ALL have there stupid asses and they all have there pluses. I joined the Army because it was best for me. You can't join something based off someone elses opinion. Make sure to do your research, talk to someone prior to signing your papers just so those with experience can make sure you aren't getting fucked by your recruiter.
Best of all, enjoy yourself the next (xx) years of your life will be awesome.