Taking into account the EU, the X-Wing is a ship under development leading up to ANH, and when the Empire tries to nationalize the company making it the designers bail and take the design and prototypes to the Alliance who then begin manufacturing X-Wings.
Y-Wings are old school ships modified and used by the Alliance... So is the Z-95 and the T-Wing if you delve further into the EU, and probably a number of other fighters are sprinkled around to various cells and squadrons who are part of the Alliance all around the galaxy.
The X-Wing is a pretty purely Alliance ship then, like the A or B-Wing which the A-Wing is prototyped and in pre-production in the immediate events right before ANH (if you again subscribe to EU) with Yavin even having a couple of the prototype design available to them. The B-Wing is developed years later as something to assist against the capital ships the Empire deploys in great numbers.
The Empire/ALliance tactics are different (again all EU stuff, but some truth from the film POV) in that the Alliance puts a greater value on their pilots and thus try to develop more costly, but more durable craft... The Empire use swarm tactics more, though they have and further develop ships as the war escalates to more greatly match (or outmatch) one-on-one what the Alliance puts out against them.
I look at the X-Wing as an evolutionary step forward in the plane's design, kind of like taking the Spitfire and making that next step forward to the Mustang... Similar, lots of the same principles and design elements, but wholly new when ultimately compared to one another. Similar to the CW3D Fighters looking like evolutionary steps forward of AOTC design. Ani's 2D fighter looks (and is labeled) a modification though... The ROTS Jedi Fighter then seems really like a whole new fighter design, not even really of the same family just the same guys flying them. The V-Wing, to me, seems more like where the AOTC style Jedi fighters went in terms of evolution, and then those and perhaps the ROTS Jedi fighters wind up converging to see the Empire's TIE fleets grow out of that... But the Jedi Fighters almost seem to go the way of the dinosaur in a way, just elements go from them to the TIE Fighter, but a whole new Fighter style and design is born and grows along with the Empire's birth.
Again dipping into EU, the X-Wing's design is tied to the Z-95 and other ships in the Incom Corp library... I figure the X-Wing we see is going to be something we see more than one of. The time elapsed, it makes sense. It's something I wish we'd seen more of with the Y-Wing, and wish we'd seen Z-95's, but I guess Lucas didn't agree.
I personally loved seeing the YW show up in CW3D at least though, in one form or another.