For Mustangs, there are three choices:
1- A2A Mustang (civil or military) without its Accusim addon
2- A2A Mustang (civil or military) with its Accusim addon
3- WarBirdSim Mustangs (B, C or D)
In my opinion, the choice #1 should be excluded, excepted if you just want a Mustang for doing screenshots and flying without caring about realism, constraints or checklists.
So that leaves the choices #2 and #3. Both of them have a high realism and high graphic level, so your choice will depend on what you are really looking for.
The A2A aircraft with its Accusim addon will be the most realistic and immersive one, thanks to the unique features brought by the Accusim module. The various aircraft systems are reproduced with a very high degree of fidelity, and this includes almost everything, for each single gauge or aircraft mecanical or electrical organ. The engine is not simply ON or OFF, it is constantly "working" based on various conditions which are handled by the Accusim module. That module also handles the wear and damage of your aircraft, down to the smallest details like the effects of worn out pumps or cylinders. And again, this isn't just working/broken. All various stages of wear are simulated, with precise impact on the behavior of your plane and its engine. And all of this is based on what you do, not just random. The way you use the plane will directly impact its wear.
The WarBirdSim Mustangs are the most beautiful Mustang models available today. Contrary to the A2A Mustang, which is limited to just one Mustang D model, the WarbirdSim packages offer various Mustang versions, and each version is modeled in a very precise historic way. A Mustang with a certain repaint will also have all of the physical details of that precise historic plane, including the amount and shapes of external mirrors, shape of the canopy, shape of the antennas etc... The level of visual details will be greater than the A2A planes, and this is also valid for the quality of the textures. Additionally, the plane has a very realistic flight model, and if I remember correctly, it also has a very good quality for the sounds. Finally, this package is also less expensive than the A2A Mustang with Accusim.
Now concerning the Spitfires, there are a bit more choices:
1- A2A Spitfires (Mk.I and Mk.II) without its Accusim addon
2- A2A Spitfires (Mk.I and Mk.II) with its Accusim addon
3- JustFlight/AH Dunkirk Spitfire Mk.I
4- RealAir Spitfire
Here again, the choice #1 can simply be excluded from the start, for the very same reasons as the Mustang above. The choice #3 might be a better and more recent alternative model.
Just like the Mustangs, the choices will depend on what level of features and realism you are looking for, and also which model of Spitfire you want.
The choices #2 and #3 concern the earliest variants of the Spitfire, while the choice #4 by RealAir is a later variant (long nose).
The A2A Spitfire with its Accusim addon offer the same features and level of realism than the A2A Mustang described above. Additionally, it also allows you to choose a few variations for the planes:
- for the Mk.1, you can choose, in flight via a 2D panel, the kind of prop (wooden two blades or two-speeds metal tri-blade) and the kind of canopy (flat or bubble)
- for the Mk.II, I don't remember what can be chosen, sorry

Each choice will immediately reflect accurately in the cockpit and on the plane behavior. The simulation of the various prop types (wooden, two speeds or constant speed) is quite accurate with a very nice video on Youtube from A2A to explain the differences. You also get the appropriate controls in the VC based on the prop you chose.
Depending on the model, the A2A Spitfire has an electric starter and manual pumped landing gears, or a cartridge starter and electric landing gears (all properly simulated with specific VC controls). The engine cooling system is also nicely modelled, and extended gears or flaps will also influence the (bad) efficiency of the radiator. As a consequence, you'll have to carefully avoid all airports with taxi times longer than 5 minutes, or your engine is going to overheat
The Dunkirk Spitfire from AH is the most recently released model. It is also a Mk.I if I'm not mistaken. Compared with the A2A, the 3D model is a bit nicer and some textures have a very nice photorealistic effect. The flight model, sounds and graphic features are probably as good as the A2A one, but there are less features. Of course, the price is lower too, so it could be a very good choice based on your budget or the features you are looking for.
The RealAir Spitfire is a later variant of the Spitfire (beautiful long nose, two radiators under the wings) but also the earliest released Spitfire for FSX. Graphically, it still looks very good fortunately, but its strong point is the flight model, from what I could read on the forums. It seems this model offers one of the most enjoyable behavior/flight feeling for the pilot. Concerning the realism of the systems, I know that the simmer has to be careful about the engine parameters (overheat and/or overtorque?) but I don't know much more. Probably other simmers will be able to go further in details. Anyways, if you want a later version rather than an early one, this Spit is your only choice, but it's a good one so you shouldn't fear
