I think you get more copper. If I just walk up to a copper node and start from the center working down, many times the center of the node is filled with rock. As far as time goes I agree, it might even take longer lol. But since I "think" I am getting more copper I continue to do it the "explode node" method.

I guess to fully experiment I would need to do both ways on multiple nodes and record the results, but I would rather build my copper palace first.