An external stem, if of hardwood, will provide more ding resistance and is, in a way, able to be sacrificed and then replaced if necessary. Also, an important point, I think, is to remember that you are not building a spear and a nice soft radius on your bow is a safer than a knife edge. This radius (IMHO) looks nicer when done on an external stem. Try black walnut, when you get done with it you will want all of your boats to have one.

good luck jim