Army FW
You'll probably hear a million different versions on this subject, since the status of the Army's FW aviators has varied greatly in the past. Only those who are current FW aviators have the up-to-date info for sure, but for your sake I'll put in my .02 cents...realize that I was an Army aviator in 1999, and I flew RW assets. So my view is probably dated.
"Back in the day", commissioned officers could become FW pilots...there weren't alot of them, but there were some, and it was possible to attain that goal. But that was also in the day of commissioned instructor pilots as well...those days are long gone. You may find a few old heads who are IPs and former FW pilots, but things evolved over time.
When I was an Army aviator, pretty much only warrant officers flew FW. There were a tiny handful of brand-new WO1s who got a FW slot out of helicopter flight school....about 2-3 a year. For the rest of us, you pretty much had to be flying a legacy aircraft (ie, an airframe with no future...UH-1, AH-1, OH-58A/C), or have a significant amount of RW time under your belt (about 6 years or so). The legacy guys could switch to FW because they didn't have a home community (ie UH-60, AH-64, CH-47, etc) to belong to. Everyone else had to stand in line.
If you aren't aware of this yet, commissioned officers are only good for a few things in Army Aviation....commanders and battalion/brigade staff. That's it. And since most FW units started getting CW5/CW4 folks as commanders, that pretty much sealed the fate of the commissioned FW pilot.
Nowadays, I hear there are no longer any FW slots out of IERW (helicopter flight school). There are very few legacy airframes out there. So you can count on going to a flying community and having to stand in line for your FW slot. And you might never get there. FW slots are typically viewed as a "reward" for those guys who were excellent performers. And few of them stay FW after they get the qual...they will go fly FW for a while then get sucked back into helicopters at some point, unless they get out of the Army or retire and become a DAC (Dept. of the Army Civilian....civilian FW pilots).
Bottom line...don't go the commissioned route if you ever want to fly a C-12 or a UC-35. And don't expect an easy route into the cockpit of one even if you go warrant.