Not sure about Yamaha's saddles but I find plastic saddles to produce a brighter/thinner tone while bone generally produces a fuller and more robust tone that isn't necessarily louder. I've played several Yamaha GC guitars and they all have that signature yamaha sound which can tend be quite bright and sometimes "plasticy", so I'd probably prefer a bone saddle to balance and round out the sound.
We can only wonder why yamaha didn't use bone for the GC22, while the GC32 onward does use bone. I'd imagine cost cutting is a part of it, as well as giving customers a greater insensitive to spring for higher models such as the GC32. It often just boils down to marketing tactics. They obviously wouldn't use the "urea" saddle if they felt it was superior to bone since they reserve bone for their highest models. I've played a GC32 and liked it quite a bit more than the GC22 because it had a more refined and attractive tone, but who really knows if you can attribute that partially to the bone saddle.
If you haven't yet developed the technique and tone production to really tell the difference, then it probably shouldn't matter that much to you anyway. If I were you I'd just switch back and forth between them as desired until clear differences start to surface.
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