I got an xm for christmas last year and installed it ans paid for a 3 month sub. After about a month and 1/2 I went and bought a Sirius tuner and paid for another 3 month sub because the xm was terrible for
my listening tastes.
I listen to a lot of 80's rock, hard rock and alternative and there just isn't much to choose from in those genres on the xm dial so there I was with 100 channels of crap and nothing that I cared to listen to.
My take on the two is as follows.
Pop, R&B, and rap all go to xm.
Rock, alternative, sports, and talk to sirius. (Go Bubba!)
News, jazz, and international are about the same.
As for SQ, I don't see any difference at all. Both have popular channels that aren't compressed very much, and off the beat channels with heavy compression. For my tastes the SQ of the sirius is just as good as the xm was. It would be even better if I used a line out to a modulator but I switch vehicles too much to permanently mount the unit so fm it is.
Plus I listen in the work truck where taking the dash apart isn't an option. I just removed the outside antenna from the truck and there is zero interference. I wish I could say the same for the wife's 4runner though. The antenna's in that pick up all kinds of crap and I have to switch channels about every 30 minutes. I'm probably going to get a cassette adapter or something for it.
So for the skinny on the above, I have a sirius and love it to death. I won't drive down to the corner store in my wife's truck without grabbing it. terrestrial radio is really that bad. You just don't know it yet.

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