Sirius in the Dodge - no dropouts ever, except getting gasoline under a metal roof, but the music channels - except classical channels - are barely passable as lowbitrate MP3's. Not a nice sound, even after playing with the 3 band EQ.
XM in Nissan- audio acceptable - not the horrible MP3 sound, but bass is lacking and dropouts in a few regular areas (under railroad bridge & large tree-lined areas). Same deal, play with 3 band EQ to get acceptable results, but only the classical sounds decent (and CC channels). My boss' XM in his big Buick arsehauler does sound fantastic though.