They should sound different. The first major influencer of a guitar's sound is its construction (size, shape, bracing). The second major influencer is the top wood. Spruce sounds quite different than hardwood tops like koa. It should sound more open and a bit louder. The third influencer is back and sides wood, but this one has less effect than the other two. I'm not a huge fan of koa-topped guitars, but I do own a spruce topped koa GA that I love. Give it a try. It's not duplication in my book.