Thanks, everyone, for the information. These are my first Japanese chisels so I'm just leaning.
I'll try a metal hammer - I understand the advantages - but it's hard to overcome the teaching of not using a metal hammer on a wooden handle.
Dave, those hammer weights seem heavy to me. I use a 10 oz (about 300g) mallet for the work I do (carving, dovetails, tenon trimming, etc.) and am pretty satisfied with it. I'll try to buy or fabricate a 500g metal hammer for mortise work and see how I like it. Anything heavier would just be too heavy for me. I guess that's just part of the "Different strokes for different folks."
Mike