Then you didn't read MY post in that thread.
I've used both XM and Sirius, though in 2 different cars. I have Sirius with the D3.
Better interface for D3 is hands-down the XM. The Sirius interface is absolutely terrible. You get a 1-line display (scrolling), no category/channel list (only up/down buttons), and the presets get saved as "Preset 1, Preset 2", etc. You have to remember which station you saved to each preset. Read the D3 manual to see what the XM interface looks like.
I think Sirius has an edge in sound quality. After listening to XM exclusively for a period of time, I used to think my stereo components were failing or my head unit settings had been changed. Then I switched back to a CD or iPod and everything sounded awesome again. That car has a decent aftermarket system installed. I think the Sirius sounds better on a stock system than XM on my aftermarket. XM seems to cut the bass and treble frequencies, resulting in a flat, almost muffled sound. Sirius is still not close to CD-quality, but it's better than XM and much better than FM.