In his PM to me nwhitehorn asked about discs vs drums, especially regarding safety. I'm answering here because others may want to chime in. Here's my opinion:
The drums, especially when polished, look a hundred times better than discs, so they look better on hardtails and customs, which are all about looks, and they have the correct vintage look if your look is more stock.
Discs have way more stopping power than drums. Regarding safety, the problem is when you have a disc front, and a drum rear, which is how most XS650s are (and my preference). In a panic stop, you will grab a big handful of front brake, the drum rear will be doing very little, and you will go down. So, a disc rear is a lot safer in a panic situation. The problem is made even worse due to the excessive dive of the front forks. I am completely sold on doing the Minton Mod to your forks, and adding one ounce of fluid over the specs to each leg -- together this makes the stock forks behave as they should, much safer. I also think that once a year everyone should go to an empty parking lot and practice panic stops.