I don't have the best ears but can hear the differences. I have one spruce top, the Ohana Vita and LOVE that one. My mahogany ukes are old sopranos with wood saddles, they are also loud but not as, and warmer as noted in previous posts. These ukes also have that plunk quality that I think has a lot to do with the wood saddle, because my old Kamaka has it too, plunk and also a woody sound. All have good sustain.
I have an old koa soprano too, not much difference noted between it and the hog, both are loud and warm, both are 1920-30. I have a cedar CBU concert that is very brash and loud but still warm, with great sustain. My Kanile'a koa tenor is low 4th G and sounds more guitarish to me (so does the mahogany bari) because of that, and seems quieter actually with it's bone saddle.
Bradford is working on a custom pineapple for me, go check out the luthier section to see pics of the bear claw spruce top.
I think a well made spruce top would be a welcome addition to any collection.