I have a Sirius Sportster unit. The one you should get should depend on what your listening preferences are.
AngryJohnny is right, XM does have deeper playlists. This can be a plus or a minus. You may hear the same song more often on Sirius but I found often on XM, they are playing some random stuff that I don't want to hear. Its preference.
XM generally had the hardware edge but I think they are pretty close now. XM does have a larger subscriber base so that is a plus. However, that could be because they used to be cheaper than Sirius and had a head start. Over time, they had to start offering all the same services as Sirius did (no comercials on music channels, no pay channels, free streaming audio, etc.). After doing this, they raised their price so XM and SIRIUS are equal in that department.
Either way, you can't go wrong. Go over to CC or BB and listen to both and choose for yourself. If you need MLB, go with XM... If you need NFL, go with Sirius. Also, like XMFAN.COM, there is SIRIUSBACKSTAGE.COM. There is a lot of info out on these sites but be careful when reading comparisons. They are often very one sided. (whatever I have is best and the other one sucks mentality).
Anyways, here is a link to my Sirius install pics. I put it in the cupholder and ran an aux power supply, PIE adapter, and antenna thru a hole in the cupholder (i.e. clean look).
Good luck!
http://www.toyota-4runner.org/showthread.php?s=&threadid=5208&perpage=15&pagenumber=2