In the end, Wake is just too small to be defensible, if any enemy really wants it.
And in 1941, the Japanese really wanted it, and spent what it took to take it. Had they realized what its defenses were, they would have detailed a more powerful force in the first place.
The United States in 1943-45 did not really want it that badly, because they did not really need it to execute their strategy. So they isolated it and left it alone, barring occasional air raids. In this sense, Wake is more valuable to Japan than it is to America.
Guam is obviously harder for the U.S. to supply or reinforce once hostilities break out, but it's even more urgent for Japan to take, and even easier for America to defend. The immediate advantage to the U.S. is as a base for submarine operations, but the Japanese also know that the U.S. is developing heavy bombers that can reach Japan from Guam.
I think it was Calbear's Pacific War Redux timeline that showed how Guam might have a fighting chance to be held by the U.S., though it clearly depended on an early point of departure (mid-1940) to make it so.