What is the best route to become a military pilot? Warrant or 2LT? Which branch?

During army flight school they say "choose the mission, not the airframe". I think that needs to be the same for choosing which service she wants. Probably the first decisions she needs to make are:
1) Does she want to be Active Duty or Guard/Reserve
2) What type of missions does she want
3) What type of airframe does she want (type meaning fixed-wing, rotary-wing, or tilt-rotor)

If she wants to fly helicopters, then it's a pretty easy decision on the service (from my perspective), Army, unless she's looking to do sea duty. Army has by far the most extensive helicopter fleet. If she's looking to do tilt-rotor, i.e. V-22 Osprey, then again, it's a relative easy choice because only the Air Force and Marines have tilt-rotor craft. (Beware though, tilt-rotor time only counts for tilt-rotor time, i.e. not FW or RW, so it's fairly useless in the civilian sector... at least for now) If she wants fighters, then AF, Navy or Marines

Army: Predominantly RW and predominantly a local area missions. Army FW is predominantly MI on the AD side and predominantly OSA on the NG/Reserve side (more Reserve than NG... and you'd be surprised how much flight time they actually get. Most NG/Reserve average at least as much as AD, at least in the Army). Army is also now "FW for life" so if she assess as a FW pilot there is no chance to go back to RW (like there used to be) and vice versa, you can no longer apply for FW later down the road if you are a RW guy. Army also has the 160th SOAR (which I imagine is open to females now since they opened everything else up).

Navy: Sea duty. Unique aspect that only they can really offer. I have done some deck landings on Navy ships before, but it's definitely a cool little extra as an Army pilot and not a "norm" for our mission. Also, more of a regional mission. They have larger planes than the Army but they're still staying more or less in one region/geographic command.

AF: Potential worldwide missions, depending on the airframe. The only service with true "heavy" aircraft. Also unique aircraft like the U-2, B-2, and B-52.

Marines: Uncertain as to their actual mission so I defer to any Marine pilots out there. I think the largest aircraft they have is the C-130. Mostly fighters and helos as far as I know.

Coast Guard: Pretty much all rescue operations. Certainly the smallest fleet of all the services. Plus side is you're staying stateside. Down side is you don't really have the opportunity to deploy or be stationed overseas. I've heard though that because they're so small pilot's can be retrained on different airframes when they PCS. (Are there any CG pilots out there that can confirm this?) If that's true then that's certainly an advantage as opposed to the other services that you are trained on a specific airframe and pretty much stay that for your career, save for any special assignments that require special training.

As far as Warrant vs. RLO, only the Army has flying Warrants anymore. As others have said, at least on the Army side, if her goal is to stay in the cockpit then Warrant is the best choice. RLOs get little flight time after CPT, and those really only get it if they are lucky enough to be selected for BN/BDE command of an aviation unit. If they are selected for command of a unit that does not have organic aircraft or are doing staff work outside the unit then they will likely not be able to fly. Warrant also has the benefit of applying for the WOFT program as a civilian, so she would know if she was accepted to flight school before ever joining the Army. In the Guard and Reserve she would also already know what aircraft she was going to fly before going to flight school (for WO and RLO alike).

If she's looking to fly in the civilian world after her initial commitment then I think any service would be good because the majority of your flight time in all services is toward the beginning of your career before you "promote yourself out of the cockpit". Employers prefer recency (within the past 12 months) in the cockpit when they hire.

Well, that was much longer than I intended, but hopefully it gives some good thought points. I think each service is strong in certain areas. At the end of the day, they type of flying you want to do is more important than what airframe you are flying because that is what will make you either love or hate your work. Any other questions, feel free to ask.

Speaking from someone who has been in the Air Force and the Army, here is my advice. First, she should decide what her ultimate goal is? Obviously she wants to fly, but does she want to be a career military person and retire? Does she just want to do the minimal amount of time in the military and then fly in the civilian world? Does she eventually want to get married and have a family. In the Army, being a Warrant Officer is probably the quickest way to guarantee a full career of flying. However, the Army is very deployment heavy and doesn't leave much room for families. The Air Force doesn't always guarantee a pilot job. Some have hopes of being a pilot, but end up being a flight engineer or other flight support job, but not actually a rated aviator. Air Force treats their people better, they have nicer bases, and deploy for shorter durations than the Army. Looking back on everything that I've done, it would seem like flying for the Coast Guard would be the ultimate. Much fewer deployments, if any at all. I can't speak for any Navy or Marine flying. Being a female, I can see Marines and Navy not being quite as female friendly as the Air Force or Army. Not to say anything bad about the Navy or Marines, but if I was locked up on a boat with a bunch of dudes all the time, I'd probably be dry humping anything that resembled a female. I think the Army and Air Force do a good job with all the SHARP stuff. I saw many females successfully navigate Army Warrant Officer Candidate School and Flight School. That's my $.02 and feel free to ask me any questions.

If she wants to be rotary, the Army is a good choice. You can apply (it's a long process) with no prior service, no prior experience, and if selected, attend BCT, WOCS, and flight school which is broken into about five sub courses. With that she's at least guaranteed the branch if selected. She could, depending on how she does in flight school, select fixed wing. There have been slots but usually go to top of the class. Here is some information on the application process. Best of luck.

http://www.usarec.army.mil/hq/warrant/prerequ/woft.shtml