As far as I know, the XM and Sirius antennas are not interchangable on the hideaway units, nor the dash mount units. I had the Sirius package in my Truck when I bought it. I already had XM and had no intention of changing. I ripped it out when I installed my new head unit.
Just as a heads up, if you are planning on removing the Sirius system from your truck, the antenna is held on through a hole in the roof by a stud and nut. I removed my antenna, ran both my XM and GPS Nav antennas through the hole, which was protected by a rubber grommit, then siliconed closed. It sure was a PITA pulling the cable from the Sirius unit back to the Radio.
My suggestion is to buy the Skyfi 2, the car mount cradle, and the inline FM modulator (plugs in between the antenna and radio, run a power and ground, plug the other two wires into the cradle). You can mount a second cradle in another car and shift the two units between the vehicles and only pay for one subscription. I do this between my Truck, my Jeep, and my boombox in the garage. You can also listen to XM on the computer through the internet.
Sirius SUCKS!! Stern BLOWS!!
XM keeps signing up new stations and shows constantly. My favorites are XM202, The Virus/Opie and Anthony & Ron and Fez, and XM150 XM Comedy.