Sirius or XM

I just installed an audiovox xpress (xm satellite) for my '05 av.  Used the USA SPEC to direct connect to my factory h/u instead of using the fm modulator.  I currently have the xm antenna mounted inside on windshield.  The reception is very poor (sound quality is definitely not cd quality).  I was wondering if its the location of the antenna (should i mount on outside of vehicle?) or if Sirius provides better sound quality.  Even with the full strength signal on the xpress unit (all bars) i am still not impressed with the sound.  ANy suggestions. Oh, i do have Onstar (dont know if that matters)

defined impressed?

With digital...it is pretty much on or off...by nature...

it can fade in and out with crappy antenna location...

but all in all the sound problem may be related to you Adapter....

The choice is more of a programming preference.

Both XM and Sirius have some compression. More like a good MP3.
Realize that the quality of the D to A converter the brand/model uses, more reflects the audio quality than that of the satellite service of your choice. At least as far as your ears can discern.

If it's bad enough you can hear, there is a problem.