(These questions are assuming both cars have a manual transmission)
In terms of how fun they are to drive, which car is a better choice and why? A Subaru STI or a Ford Gt Mustang (V8 engine)?
In terms of reliability, which is a better option and why? Subaru STI or Ford GT Mustang (V8 engine)?
In terms of both, which is a better option and why? Subaru STI or Ford GT Mustang (V8 engine)?
I would say the mustang with the V8 would be more reliable because those 5 liter engines are pretty solid. The Subaru has a 4 banger boxer engine which is not as reliable and blows the head gasket. The mustang would also be faster than the Subaru.
Turbocharged Subarus haven't had systemic problems with their headgaskets. (For that matter neither does the current generation of normally-aspirated boxers.) However the Mustang V8 would certainly be longer-lived than a heavily-turbocharged 4-banger.
2018+ Mustangs have engine and trans problems. Google the Coyote tick and oil consumption for a start.
Can't really recommend either of those brands.