QUOTE(BrADmatt @ Jul 2 2008, 02:01 PM)

I was wondering which way I should file the N-400??
I have lived in the US for 28 years with a I551 resident alien card, I have also served in the military (both Army and Marine Corps 1980's), and I have been married for 18 years to the same person.
Should I file under marriage or military service??
If you file based on the marriage, you'll have the burden of proving your spouse's citizenship, plus the fact that you've been living together in valid marital union for the past three years. So it'll mean at least more paperwork you'll have to send in, plus potentially more questions at the interview. Furthermore, in the unlikely event your marriage ends due to death or divorce at any time before you take the oath of citizenship, you'll be ineligible to file based on the marriage. In your case, I'm not sure if that would mean starting over or just having them readjudicate the petition without taking the marriage into account.
Those probably aren't really big issues, and seem very unlikely to cause problems, but since you're eligible without the marriage, why complicate things in the least by bringing the marriage into the picture? Just file based on five years as an LPR.
The military service thing would similarly require you to send in more documentation to support your claim. If you have the documentation handy and feel like sending it in, it doesn't seem like there's much chance it could hurt anything, so follow your gut on that one.
But if I were in your situation, given the choice, I'd probably take the simplest, most straightforward route that required the least documentation. In other words, I'd check box "A" on the N-400 part two, to apply based on five years as an LPR.
But you are eligible to check box "A", "B", or "C", so the choice is yours.