LiveGold15, your advice is actually completely wrong. There are a significant number of Marine aviation slots set aside for USNA (roughly a quarter to a third of the Marine slots). This isn't a rumor, this is direct from my company's Marine mentor. So...yeah.
The Marine Corps offers three different kinds of contracts for officers: air, ground, and law. OCS can compete for all three, USNA and NROTC compete for air and ground. What that means is that when you commission you attend TBS knowing very generally what you're going to be doing. Ground Marines are assigned an MOS at TBS (i.e. infantry, artillery, logistics, etc.), and air contract complete TBS but already know they're going to flight school. They aren't assigned a community until flight school.
As for getting rotary wing, it's a safe bet that in the Marines you stand a good chance of getting helos. Traditionally, the Marine aviation communities have been helos, props (C-130s, and it's a small community) and jets with helos being the largest. This is changing with the introduction of the Osprey replacing the CH-46 medium lift helicopter.
Helo communities within the Marine Corps have a broad range of missions that are all awesome (which is partially why I want to fly helos in the Marine Corps). Basically, there's:
HMLA (Hueys and Cobras): Provide close air support, CASEVAC, inserting/extracting Marines, and carrying small piles of stuff around.
HMH (CH-53s): Heavy lift support, from inserting/extracting Marines to carrying huge piles of stuff around.
HMM (CH-46): Medium lift, such as CASEVAC, inserting/extracting Marines, and carrying medium piles of stuff around.
One thing to consider is that all pilots in the Marine Corps are commissioned officers. In the Army, a certain number of their pilots are warrant officers. In Army units (one of the Army guys correct me if I'm wrong, but this is what I've heard from Army pilots), the warrant officers kind of get all the fun because the commissioned officers have to spend a lot of time working on their "ground jobs," a secondary position they do when not flying. In the Marines, everyone's kind of in the same boat as far as ground jobs.