The advice on WHO can put promises in writing BEFORE you enlist is the best advice, if a particular skill training is your goal. It used to be that ONLY the Army could actually promise you a specific training prior to enlistment. The others were mostly contingent on passing additional tests after enlistment.
If you are looking for mechanic training--most of them can provide that. Aircraft mechanic is a limited private industry field. I have a son that was an attack helicopter mechanic (Cobra). There is not civilian counterpart. If you are really interested in mechanical work, I would suggest something in the diesel mechanics field--the money is better in heavy equipment repair.
Generally, the living conditions are best in the Air Force, but advancement is usually faster in the Army. (I leave out the Navy and Marines because I was never interested in the possiblity of spending a lot of time on a ship--just think what you miss on a ship
well unless you like that
) Of course, the faster advancement depends on the total number of people in the particular service with a certain skill and the actual working level of that skill. Like, there are a lot of Infantryman in the Army and promotion is pretty good, but there are very few Broadcast Specialists, so you stagnate.
The advice on long term goals is also good. The military is a HARD life for a family. Very difficult for an officer and damn near unbearable for an enlisted man.
Joining the Reserves or National Guard gives you other options. You are a part time soldier/airman most of the time, but still have the option to go on active duty for a longer term if there are openings in your field. Of course, as you have seen during the Iraq/Afgan Wars, many of these citizens soldiers have spent a lot of time on active duty. In the Guard or Reserves your military pay is not significant and you will still need to hold down a full time job. Full time work AND military duty can be a drag sometimes. While you do gain some educational benefits--check them out carefully. Work, military duty AND school can be a killer. Talk to your g/f and other Guards/Reservists about this part.
I also spent 25 years in the military and don't regret a minute of it and would do very few things differently. But---it is not for most people.
Lee