Here is my take on the subject:
F-35
The F-35 represents the most advanced fighter jet on the planet. It may not have an edge over the F-22 but still, it has a host of systems that is a generation more advanced.
Receiving the F-35 will give Turkey an uncontested aerial dominance in the region (with the exception of israel). Turkey will have the upper hand in aerial conflicts with most countries in the region such as Iran, Syria, Greece and even Russia. It also provides Turkish companies with industrial participation opportunities woth upwards of $12B.
But most importantly, I dont see any realistic replacements for the F-35. The TFX is an air-superiority-oriented fighter and so is Su-57. The Eurofighter Typhoon is a 4th generation fighter and it will not be really an upgrade over F-16s. There are no other 5th generation ground attack fighters in the world.
The F-35 also makes the dream of an aircraft carrier possible for Turkey. At the moment the only possible choice of a carrier-based fighter jet for TCG Anadolu and Trakya seems to be the B variant of the F-35.
S-400
The S-400 is said to be the most advanced air and missile defence system in the world. It has an unrivalled range of 400 km which makes it an outstanding asset. The S-400 will also be a safeguard for Turkey in case of a conflict with NATO or European countries such as Greece. Because even if the rumored “off-switch” in American weapons is not true, the US could cut off supplies of spare parts in case of a conflict that they dont like and we would be sitting ducks without the S-400s. We know this too well from the Cyprus Peace Operation
However, I believe the S-400 can be replaced rather easily. We have our own solutions such as the Hisar series of SAMs and the Siper system which would make up for the loss of the S-400 just fine. Worst case scenario we could acquire the European SAMP-T system.
So in conclusion, I personally would prefer the F-35 for the following reasons:
- It is irreplaceable
- It comes with $12B+ of industrial participation
- It has dual purpose (An advanced jet + carrier deployability)
- Purchasing the S-400 means severing of ties with the US which we very much need for our defence projects as well as for their investments