I have Sirius and REALLY like it. The music selection is very wide and varied, and their other channels are interesting as well (comedy, news, talk, sports, etc).

I've riden in rentals/friends cars with XM and it never clicked with me - mostly due to music choice (although it's been 4 or 5 months, it's possible it's changed by now).

As for satellite vs FM - no comparison. I never use FM anymore, can't stand it. Drives me nuts to get in my wifes car without satellite...

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1995 E300D - 225,000 miles - White Exterior, Grey Interior - original owner