which pistol should i use to carry on duty?

Your employing agency will provide specific guidelines for what can be carried. I assume Air Marshal's will have some of the most strict requirements due to pressurized cabins on aircraft. Ask someone in the agency for a list of authorized weapons.

Why would you want to carry such a huge caliber weapon as a backup? Your dept will most likely issue a .40 caliber or .45. You are going to have a tough time concealing something like that while also providing comfort. I would recommend something like a .38 caliber on the leg.

As a side note, I think it is funny that you state that you are going to work in federal law enforcement when you graduate but you don't list an agency. Have you applied? Do you even know what it takes to enter federal law enforcement? If not, you better apply now. It takes TONS of time for a Top Secret Clearance which is required for federal law enforcement. I'm sure since you state you are graduating that you are leaving with a Bachelor's Degree. If not, you need to finish because you are competing against people with 4 year degrees. Your maturity level and lack of research on the topic indicates that you aren't old enough to posses a 4 year degree. Reading through some of your other answers on Yahoo! Answers in which you state that Reckless Driving is a felony makes me think that you won't get very far in the application process. Do some research yourself. It is nuts to come here and ask what kind of gun to buy. This will be different for EVERY person.

You will be issued your firearm. Which one depends on which agency.

You will probably never carry an FN FiveseveN. Neither is it likely you will be issued a 1911 unless you're on a Tactical team of some sort. Though it is an excellent choice for concealed carry purposes.

Plan on a Glock, or Sig if you're going the Agent route. If you're going "uniformed" Federal Law Enforcement, you'll most likely be issued a Glock. But each agency is different.

For personal carry. Carry what you can reliably and consistently, present smoothly from concealed carry... and place rounds center mass at 7-15 yards. You might be great with a 9mm but suck with a .45. You might drive tacks with a .40 cal but find that a 10mm is too much. You may have big hands or small hands. You might find a particular frame hangs up when drawing from deep concealment. You may find another is more suitable. Shoot what you're comfortable shooting and carry only what you're proficient with.

Long and short of it is... all you will get here is opinions that fit the answerer. No one knows you or your capabilities. So your question can't really be answered properly.

EDIT: You said: "* AS A FEDERAL OFFICER YOU ARE AUTHORIZED TO CARRY YOUR OWN PERSONAL SIDE ARM, WITH OR SANS DEPARTMENT ISSUED SIDEARM - fyi"

I'd love to see a link to a federal website attesting to that fact. Doubt you will be able to produce one. But I'd love to see it. I'm sure that is true "OFF DUTY" but I'm afraid you might be disappointed otherwise.

One other little... observation. You haven't even gotten a job yet and you're preoccupied with what type of gun you are going to carry. That's a little premature... if not a little "immature." Get your job first. Then worry about the specifics. Sounds like you just want the jock gear so you can look cool. If so, you won't make it past the interview. Any cop here will agree. You're focusing on the wrong thing.

Most federal agencies use .40sw. Its a powerful large caliber round, great penetration and stopping power. I know its the FBI's first choice. A lot of guns use the round as well.

.45 ACP is an excellent round as well. 1911's make excellent sidearms. They rarely jam, can shoot in any condition, and have good power and penetration, but not as much as the .40sw. There is also a large amount of parts and upgrades. There's a reason many sport shooters use them.

Both are widely used and supported, so that's a bonus.

.45 caliber