XM has far better music programming IMO.  Certainly, if you're 30 or
older.  If you like the current garbage, you might prefer Sirius.

Sirius DOES NOT HAVE NBA any longer.  Also, XM has added NCAA
conference football and basketball for ACC and PAC10.  Rumor has it
they are considering adding others.

Sirius playlists are much closer to "FM" -- 300-900 songs.  XM's are
much longer in many cases; for example, Deeptracks (my favorite XM
channel) has 4000 songs on their playlist.  You don't get the
repeition from XM you do at Sirius.

If you want NFL games on the radio, Sirius is the place to go.  I have
both, but much prefer XM.

Thanks to all for all of the opinions. I'm going to check further but, right
now, those NFL games sound real good over on Sirius!

I have XM in my FX-35.  It is fantastic.  The sound quality is far
better than in the Pathfinder I had before (which was a wired
modulator install).

I'd go for the dealer install.  XM is a far superior service IMO (I
have both).

This is really not the "big" difference.  Both services have dead
spots, both have reasonably good reception where ever you are.  In
general, the antenna you're using is more important than anything
else.  XM has 4x as many ground repeaters.  Sirius appears to have
more problems with units in homes, as their satellites are constantly
moving with respect to any location on the ground, whereas XM's
aren't.  Both of XM's satellites cover the entire United States,
meaning if you can't get one you can usually get the other.  Either
service will cutout if you're under an overpass for more than a few
seconds, UNLESS there is a nearby terrestrial repeater (XM has 800,
Sirius has 200).

Basically, reception is about the same, just with different nuances.
More people complain of dropouts with Sirius than with XM (I follow
this stuff closely).  But either is far better than FM.

The "big" difference is in the music.  XM's playlists are huge, Sirius
playlists are pretty much like FM.  On XM you'll hear stuff you would
NEVER hear on Sirius or FM.  But again, either is better than FM, and
if I couldn't get XM I would definitely have Sirius.

The only reason I would get Sirius is if I thought I was going to be
on the road on Sundays and wanted to listen to NFL games.  And XM has
been really beefing up its content lately, with the addition of NCAA
games and starting their own public radio channel.  Because XM has 4x
as many installations as Sirius, XM is financially able to do a lot
more for the listeners than is Sirius.

Clear channel does not OWN XM. they at one time had a portion of the
stock.(Traded as XMSR) They now own less than a 2-5% if I remember
right.

XM uses the terrestial boosters to stop the "cut outs" from bridges
and easier recption from indoors. But these are only in major cities.

I've only had 1 cut out and that is though a tunnel and there are no
teerestial boosters in my area.

This is not true.  Clear Channel owned a small number of shares (8
million) in XM, but has hedged that interest and has been removed from
the Board.  Stern had incorrectly stated this on his show, but has
subsequently been straightened out about it:

http://www.pokerroundup.com/sternad4.mp3