I don’t think it will be either. My money is on two different long-shots. Either they will somehow convince the US to reopen the F-22 line, which is the bird they want the most anyhow, or they will go in with Korea on the F-15SE Silent Eagle.
The second choice offers a number of advantages. It will increase cooperation between the Japanese and Korean defense industries and military forces (something both countries agree is necessary) and it gives them a platform that is very familiar in a lot of different ways because the JASDF already flies the Eagle and knows it well.
The Super Hornet is a non-starter because of its known limitations and lack of stealth, while I just don’t see Japan wanting to risk alienation from its No. 1 defense partner by choosing the Eurofighter.
The final deciding factor may be the F-35B variant, which I believe (official denials notwithstanding) the JMSDF is very interested in acquiring for its Hyuga and Ise class ships in the future.
Even as a layman, to restart the f-22 line seems improbable. Japan allying itself to SK would be a sign of desperation. It’s a option but will only show major weakness. It’s Europe or nothing. They flank Europe’s enemies. That’s they’re real advantage. Japan needs a infusion of something. Alliance to Europe would be it.
The JASDF dropped the F-15SE from their F-X shortlist a month ago because they supposedly wanted a brand new aircraft.