We have to consider this carefully. The strategic value of beefing up the Japanese Air Defense Force to counter the rising power of China needs to be considered. Without a clear balancing power in the region China will likely be more dominant in Asia which is likely to hurt our interests.
At the same time, our stealth technology is quite an investment. Any foreign power (including Japan and Israel) could easily compromise our technology. And with the growing industrial might of China, they will soon be able to fund a military as large as our own. Losing key pieces of technology would mean that they wouldn't have to fund the expensive R&D like we do.
It is a tough decision. I am leaning towards selling to Japan but only with strict technology assurances and increased security. If we fail to counterbalance China we may never get another chance to do so. And the second half of the 21st century might have China as the major superpower potentially with India under its wing. In a second Cold War, we wouldn't be even remotely close to being able to compete militarily. As long as Asian countries do not form a type of Warsaw pact-like alliance, China will not likely ever be more powerful than the US and the EU.