Pawing thru Wookieepedia, here's how all the X-Wing models are supposed to differ and what era they are seen:
X-Wing Subtypes
T-65 = base (Rebel Alliance)
T-65A2 = Twin Suns Squadron (New Jedi Order)
T-65A3 = Rogue Squadron; improved shields, targeting computers and lasers (New Jedi Order)
T-65AC4 = faster (like A-Wing), heavier cannons, more durable, more torpedoes (compete with E-Wing release) (New Republic)
T-65B = faster (Rebel Alliance)
T-65BR Recon = long range sensors, no torpedoes, self-destruct (New Republic)
T-65C-A2 = rare (Rebel Alliance)
T-65D-A1 = no astromech, more maneuverable (New Republic)
T-65 Advanced = heavier (larger hull), (Rebel Alliance)
TX-65 Trainer = 2 crew, no weapons (New Republic)
Tandem X-Wing = 2 crew, heavier armor (New Republic)
T-65XJ = more shields, better maneuverability, better targeting computer (New Republic, Galactic Alliance)
T-65XJ3 = more shields, better maneuverability, better targeting computer, more torpedoes, R7 astromech (New Republic, Galactic Alliance)
T-65XJ5 = more shields, better lasers than XJ3 (Galactic Alliance)
T-65XJ6 = Hardpoint Squadron (Galactic Alliance)
T-65XJ7 = improved shields, improved weapons (Galactic Alliance)
Stealth X = cloaking, Jedi only (Galactic Alliance)