Marine reservist grunt here, who happens/happened to be looking into moving on to Army aviation through the warrant route. Here is my take on the Aviation thing first... Great gig if you can get it. I do intend to pursue it but figuring out where my fiance will be going to med school, if/where I will be going to law school, all has to come first. The process seemed to be much easier for prior service and here is why. I am finishing up 6 years in the Marine Corps reserve. I have 1.5 years IRR time at the end of my contract. If I were to try to do the warrant aviation program for Army guard I could lat move to an army guard infantry unit and only owe them a 1 year extension into my IRR time, and not need to retrain as I am already a Marine Corps grunt (doesn't work the other way around). In that time I could submit my warrant package. As I understand it if I had no military experience I would have to sign at least a 4 year commitment to the guard, and if the aviation thing never materialized, I'd be stuck in whatever job I signed for for the duration of my contract. For this reason, if you want to be a Marine, I'd do that first, and THEN try the army aviation thing. If you wanted to be an army XYZ anyway, I'd say just do that, and you can always put in your package. The bottom line is an enlisted MOS in the Marine Corps is virtually a guarantee once it's in writing and you've got the contract signed. The aviation program in the Army is a much slower process to get in to, will require a good deal of work on your part, and if you scrub out can land you somewhere you may not want to be. Again, I intend to try to fly for the guard... but not for the next couple years at least.
Now onto Marine Corps stuff. Going enlisted there are a handful of decision you have to make. First, active or reserve. Next, MOS. I am a reservist, all said and done in a 6 year contract I have about 2 years active duty time with 1 deployment to Afghanistan, all my initial training, and various training exercises over the summer. I have had a positive experience in the reserves but likely would have seen much more "action" on active duty... that is all changing now as we roll into peacetime however. If you are not in the process of going to school, or don't have a good civilian gig, I would advise you go active duty. You will get much more of the "military experience" and it is a potential career. Also you will have better education benefits on the backend if you choose to get out.
Once you determine reserves versus active duty, you need to decide on MOS. Are you looking to get job training, or are you looking to do something you can't do anywhere else? I would love to be a helicopter pilot, but if someone said you can pick one or the other, and that's all you get, I would take infantry. In my OPINION, infantry and other combat arms MOS's are the epitome of the military. I think you should at least do it for 4 years, if you decide to go with a more civilian applicable MOS afterward (air traffic control, mechanic, etc) so be it. Why go through the hassle of joining the military to be trained in something you can get paid way more to do in the civilian world? I'll never understand why people choose "supply" or "admin" in the Marine Corps. This is not to knock those who serve, they serve a very important role, but why the hell they would join the Marine Corps to do that I will never understand. They say every Marine a rifleman, and that's straight BS. As a Marine grunt I'd take ANY of my army infantry buddies over some non-combat Marine any day. Army and Marine Corps infantry are more in sync than Marine Corps infantry and Marine Corps admin, for example.
I'm happy to answer any more questions, but I'd recommend go Marine Infantry if you want "the military experience."
Mike