Actually Terran, the USAF did NOT get an internal cannon until the E model, and they were NEVER happy with the F-4, in Viet-Nam the A-7E Corsair II was the turning, burning little fool, and did indeed dish it out to the Migs, the Navy and maybe even the Marines, also flew the A-7 and did very well with it.. Early Air to Air missiles were very disappointing, lots of "show and blow" off the wing, but not "hard kills"..
The USAF did indeed insist on an internal gun in the F-15, F-16, A-10 (actually an airplane built around a cannon!), F-22, and now the only one of three F-35 variants to have a cannon is the Air Force A model...Gunslinger's need a gun, the United States Air Force has been very firm, and very insistent since they got "saddled" with the F-4 by the "beloved, Robert McNamara??? The F-4 was a big honking, fast hauler,and a long range BVR Interceptor, it was NEVER intended to be a turning, burning gunfighter, those external guns were very cumbersome and draggy, and required frequent bore sighting to even remain close to point of aim, not to mention on, the gunsight did NOT take "lead" into the targeting solution, so NO, they weren't very effective..
The F-15 changed ALL that up,, the Navy got the F-14, an aircraft that looked and looks very kool, but lots of initial problems, engines, stall and spin incidents,prohibited from pulling any Gs and conducting ACM training when introduced to the fleet and a real maintenance nightmare,, Boeing on the other hand is "STILL" manufacturing F-15s for very happy "upscale" customers!
but Forbin, the Gun is there to "kill", its not for show and threats,, and I can assure you that USAF are killing everybody, and everything with the F-22 and F-35A...
so I'm sure the Air Force and the Marines are on the same page on lots of levels, and No doubt the Air Force and Navy need two different airplanes for very divergent missions? actually there are 3 very distinct versions of the F-35, and everbody has the bird they say they need!