Just out of curiosity: what's the difference?
First, I must qualify my Navy experience by saying that the last time I did FCFs in the Navy was in the early 90s, so my experience might be different than how it is done today in the Navy.
To become an FCP, my CO had to assign me to a job in maintenance. In my case, it was AV/ARMs division. I then went through some OJT, flew many FCFs with another pilot, and took an informal check ride. The CO wrote a letter assigning me as an FCP. In the Army, I had to be selected to go to the MTP course in Ft. Rucker. First day in the course is a chapter 5 & 9 (limitations and EPs) test that you must score 100% on. All of the descriptions and sub items on the MTF checklist must be memorized. You must draw out the AC and the DC electrical systems by memory on a blank sheet of paper several times throughout the course. You must know all of the relays, what they do and their locations in the aircraft. The course was much more difficult than any course I can remember doing in the Navy. The level of systems knowledge required of the MTP is much more detailed than that of the FCP.
The Navy FCP has less leeway in deciding what maintenance requires a test flight and what should be done on a test flight. The Army MTPs are more involved in the actual maintenance and troubleshooting. Instead of being an FCP for one tour or part of a tour, Army MTPs do this for most, if not all of their career.
In the Navy, the FCP reports discrepancies on a test flight to maintenance control. He is trained to recognize and evaluate malfunctions and suggest a course of action. Maintenance control makes the call. In the Army, the MTP makes the call. The MTP follows through on the maintenance, ensures it is complete and signed off, then plans the operational checks and test flight required to ensure the system is operational.
The MTP must do an annual APART check ride (like NATOPS) which consists of a full maintenance check flight, in addition to his normal APART. The maintenance check flgiht portion must be done by a maintenance examiner, usually someone outside of the immediate command.
The FCP is a pilot who is very knowledgeable about aircraft systems and additionally does FCFs. The Army MTP is also knowledgeable about aircraft systems, not only knows how to perform a check flight but also knows what each step of the MTF checklist is testing. The MTP also functions as an aviation maintenance officer.
There is such a difference that the Army assigns an additional MOS for test pilots. Think of it as a 1310M designator indicating a dual rating of pilot AND aviation maintenance officer.
Ther might be Navy FCPs with more experience than some Army MTPs, but as a whole, you will see MTPs with a lot more hands on maintenance experience and they will be more involved in the quality control and production control of the aircraft in their company or battalion.
I would like to see if this all still holds true. Perhaps a current Navy FCP could chime in?