I understand your situation because I was in the same boat as you. I went Sirius and bought my OEM setup at my dealer. Here's the lowdown in detail....
All 04 Armada's and up are pre-wired for both XM and Sirius.
If you go XM...
You will need to buy the xm tuner, antenna, antenna extension harness, and after a certain VIN number you don't have to buy the tuner mounting bracket. I can't find the website that had the VIN info but I needed it for my Armada to mount the tuner in the stock location next to the Bose amp. You can mount it under the steering column so you don't have to remove the accelerator assembly, but for me I did it right and went through the whole process.
As far as your headunit is concerned, Once you hook up the satellite tuner, just keep pressing the radio button and sat will appear as the headunit will sense that you have a sat tuner connected. Befor it's connected, it will only cycle through am and fm so your ok there.
The display will show you station, artist and song title but not at the same time. If you need help PM me and I'll give you my cell number.