I have had both XM (plug and play) and Sirius (integrated with the dash). Frankly, I can't live without my satellite radio.
First off, there is not a whole lot of difference in the programming. With each, you get about 150+ channels and anything you would ever want to listen to from Music to Talk to Sports to Politics to weather. And the reception works great anywhere in the country, so long as you have a clear view of the sky. Satellite will not work in a tunnel or parking garage or your garage, whereas AM/FM will work.
The only difference is that with Sirius you get Howard Stern and the NFL network, with XM you get Opie and Anthony and Fox Sports Radio. Other than that, not a big difference. Not sure how the merger will work with programming. I have heard rumors that you will now get them all with the merger, but not sure.
Personally, I prefer the integrated, but it came with the new car. The plug and plays are fine and I would not go out and get a new car to get Satellite radio. With plug and play, you generally run power into your cig lighter, and another cord with a magnetic antenna that you just route thru your window. Lugging them around is not as easy as something built-in, but for the price, that would probably be the way to go if you don't need a new car. Plus you can move them to any car or a rental if you want.
They are real easy to activate. Just have your radio id and a credit card, and they turn them on within 3-5 minutes and you are good to go. I think they are both around $12/mo. I paid for 2 years of service just once and I got the rate down to about $9/mo, since I know I am not going to cancel.