For plug making, which is a better purchase? The belt sander with the narrow vertical belt or the heavier duty one with the wide horizontal belt?

Wide horizontal belt. This way you can use the nose of belt as well as the large flat side. Plus the horizontal one has a disk on side usually depending on brand.

Sounds lilke it's a lot more versitile. Better for furniture making as well I would imagine.

i'd go with the bigger ones. in most cases, wider is better. The ones with the narrow paper are so wimpy they will often bog down on you. I'm looking for a combo disc and belt sander myself, but they can get pretty expensive for the better ones.

You can get a delta (pretty good machine) for about 90 bux at Lowe's