uncledave,
I don't think it's harder to become an officer in the Navy/USMC if you are a Reservist, the main thing that would be a concern is that you need your CO to approve your seperation from your unit and the Reserve. Unless you are a sh*tbag or otherwise not officer material, I'm sure your CO (being an officer himself/herself) would like to see an enlisted man becoming an officer. Switching to another branch of service might be a problem, talk to your CO and explain why you picked another service.
As a Reservist, you are basically applying like a civilian, you would not qualify for MCP (Meritorious Commissioning Program) or ECP (Enlisted Commissioning Program). You need to talk to your local OSO (Officer Selection Officer), you can find them at Marines.com or call your local enlisted recruiter. For Navy, contact your Officer Recruiter in your area, again call your local Navy enlisted Recruiter as you can't find an Officer Recruiter in every town like for enlisted side.
There are advantages and disadvantages as a Reservist applying for Officer programs, it all depends on what kind of Marine(or Sailor?) you are. If you are squared-away, never got into any trouble (page 11, weight problem, low PFT score, discipline, NJP or UCMJ), have a good pro/con or fitrep, hopefully a 1st class PFT (having a low PFT or slow run time actually works against you because you should know better since you're already a Marine/Sailor). Then all these would definitely give you an edge over other civilian candidates.
On the flip side, if you got into any trouble like I mention above, it would definitely work against you. This is what I gathered from my OSO and other sources, by no means are they official, just my 2 cents. My advice would be go talk to your OSO or Navy Officer Recruiter, or your career planner in your unit. They can give you a clearer picture of your situation.
Hope it helps.
DMan