I have tried both the GEX-P920XM and the CD-SB10/SCC1 with my AVIC-X930BT and here are my observations:
1. XM interface is more attractive and supports a multi-line display
2. Logos are nice with the XM unit but become outdated with channel updates and requires firmware or hacks to keep them updated.
3. Reception on Sirius is MUCH better in my area. No repeaters in my area.
4. Sirius display is single line, but does scroll (XM does not)
5. XM presets, when set, update with the channel name, Sirius does not.
6. Switching channels is slightly faster on the Sirius interface.
7. Footprint size between the units is very similar but the antenna connectors are different - depending on the antenna you have an adapter can be used.
For me the better reception of the Sirius unit was the reason I kept the Sirius unit and returned the XM unit. The multi-line display is nice but wasn't something I used much. When I wanted more info about a song, a press or 2 on the screen gets me the same info as the XM unit. The presets that don't update with channel name when set is missed though. Logos don't do anything for me.
Regardless of your choice, satellite is so much better than conventional radio!!