I used to recommend to guys to get out as soon as possible to get the seniority number and then use the mil leave to ease the pain of first year schedules and also earn points toward a retirement. That is not an option for every service, but it is an excellent option if you can work it out without destroying your home life.
Now that I have seen the industry (albeit freight) up close and personal I recommend to most that if you can possibly put up with AD until 20 to go ahead and get the 20 first. Seeing what has happened to everybody's pensions and the wage decreases that are so widespread really makes me appreciate the fact that Uncle Sam will be sending me that check at the end of the month. That peace of mind makes a ton of difference to me.
Another thing that I have realized is that airline flying is nowhere near the glamour job I envisioned (again I didn't plan on freight, but I don't see it in the pax guys either) it would be. I also can't imagine doing this for any longer than absolutely necessary. Sometimes I am really jealous of the young Captains who are way senior to me but then I realize that this (part 121 flying) is all they have ever done and I remember how much more exciting and fun (and sometimes in a bad way) the military flying was. It was much more rewarding, particularly in hindsight. 16 years on the line will be plenty enough for me thank you very much.
That is not a slam on the young, civilian background guys/gals. It is just my observation. I wouldn't be in such a hurry anymore to leave the mil behind just to fly for an airline. I'd nail down the retirement first.
If you've already got 20, on the other hand, what are you waiting for? After all, you are working for half pay.
FJ