One thing I noticed when I was a dual sub was the dropouts tend to be less jarring. XM has sort of an "error compensation-comfort noise" style system, where the last couple of frames of received audio are echoed out into white noise, and the radio keeps track of where in the stream it was, resuming at the right place.
Sirius dropouts, on the other hand, the radio seems to just decode whatever's thrown at it, including error bits, making some ungodly, deafening noises at times. There seems to be no form of error compensation, just a jarring, off of the edge of a cliff type noise. There's also no time code in the audio, resulting in time differences in when the audio comes back, sometimes by a second or so, really being inconvenient when you're tring to sing along with the songs! The only real advantage of the sirius platform I can think of is the fact that it was designed for more than 16 letters, but XM overcame that limitation later on.
XM seems to have the superior sound quality now, being a lot less swishy then sirius, and their processing has overcome most of the problems inherent with SBR.
The lag is dependant on the channel. Try a NY based channel like the 70s and the lag would probably the other way.
Howard 100 is from NY, and it's still a bit delayed on Sirius side.