The short answer is: very different. But it does really depend on several other factors too: manufacturer, solid or laminate, how it is played, where it is played, uke size, strings, etc.
However, if you compare two solid wood ukes by the same manufacturer, and all else is equal, they are going to sound very different. The difference is much more apparent with solid wood that laminate though.
Both are really excellent classic tone woods. One combination isn't better than the other, just different. For ukuleles, koa is more commonly used for top end instruments and has been for many years. However, for other stringed instruments, maple/spruce is used far more often: think of the violin family and acoustic guitars like the Gibson J200. The more expensive the instrument, the more decorative the maple will be (flame, quilted, etc.).
I play several solid koa ukes (Howlett, Kanile'a, Ohana) and also a figured maple/spruce uke at festivals (Ohana). As for the difference in sound, I have almost given up trying to describe this in words. It is so difficult. However, very simply, koa has a warm, full, more complex, well-balanced sound, while maple/spruce has a purer, less complex, sound with more treble and less bass.
The differences between two laminate instruments is much less noticeable.
Hope this helps.