If you buy a Sirius receiver that comes with the car kit (most do these days), it will have a built in FM Transmitter. THis means you can tune your radio to, lets say, 88.1 and it will pick up the signal from the Sirius unit and you'll be able to listen via your FM radio. Not the best quality option, although not horrible.
In the '06 tC's, there is an AUX port in the arm rest console that allows you to plug in an MP3 player and listen through your stereo. You can also plug in your Sirius receiver and not use the FM Transmitter. This option is better quality sound, but you'll have to run a cable from your receiver to the AUX port, as well as the power cable to one of your outlets.
If you have the stock receiver, I would suggest you just use that. There a few features missing that makes aftermarket receivers nice, but the fact that it's built in makes the stock unit nice. Your call really.
In one car I have an aftermarket unit, in my tC I have the stock receiver. I prefer the aftermarket unit, but I like not having wires or an additional unit in my tC.
It is really about preference.