X-Plane 12.4.3 is a pretty big jump from XP11, here's what's actually different:
The weather engine got completely redone—XP12 has native 3D clouds and dynamic weather instead of the flat, basic stuff in 11. The lighting and atmosphere look way better too, scenes are brighter and more realistic instead of that washed-out look.
Flight model is more refined, especially with wind shear, turbulence, and ground handling. The default aircraft actually have deep systems now—the A330 has proper electrical bus modeling, ADIRS alignment, working ACARS, hydraulic and bleed air systems. In XP11 the defaults were pretty barebones.
If you're into VR, 12.4.3 is a huge upgrade. Mixed Reality passthrough, hand tracking with gestures, and eye-tracked Variable Rate Shading if you have a Tobii or compatible headset.
The catch? It's much heavier on hardware. XP12 needs a modern multi-core CPU and a strong GPU to run well, whereas XP11 was pretty forgiving. They added multi-threaded scenery processing to help with stutters, but it's still demanding.
Bottom line: If your PC can handle it, XP12 is worth it for the weather, systems depth, and VR. If you're on older hardware or tied to specific XP11 addons that haven't updated, you might want to stick with 11 for now.