The factory receiver from Grubbs will be the cleanest install - totally integrated with the stock radio, no aftermarket boxes, etc hanging around in the cabin in the way.
Which to choose, XM or Sirius, I believe is a personal choice. I would check both and choose based on programming. I chose Sirius as they offer more NPR stations than XM.
I also heard that Sirius works better in the northern regions than XM due to placement of their Satellites, but I have no actual proof of that.