I work for XM Radio and am very familiar with both XM and Sirius. You can add the radio to any make and model of car when you buy the kit from Car Toys, Circuit, Best Buy, etc..
Personally I have listened to XM for 4 years now and LOVE it!! We are the nations #1 satellite radio with the most subscribers, we have 170 channels (vs Sirius 120 channels), and you can go on a month to month program. (I am not 100% but I think you have to pay for Sirius by the year, so you pay way in advance.. Again I do not work for Sirius so I don't know, I just have people tell me that. You should look into it.)
I actually have a coupon for a free XM Radio with a car kit included, all you have to do is buy 6 months of service (3 up front and 3 later). The radio is a $80 value, but the coupon expires tomorrow. Please let me know and I will email you the coupon code if you want it.
Direct TV's XM is different than our actual programming, Direct TV has very limited channels there are many more channels available as a subscriber. You can also go to www.xmradio.com listen to the music online for free and see if you like it. You can also do the same for Sirius, I think!
Let me know if you have any question please let me know, I would be happy to help.