Unless the vehicles are very similar in outside size and shape and vintage, I'm not sure you can logically draw any connections. One may have a better reception than the other because it has a higher quality or physically larger antenna or the location of the antenna improves the reception of one to better than the other. To assume that the radio itself is the only factor in good reception is folly.
I know this, and you are talking about rather small differences. One BIG difference is that Sirius radios have 3 tuners (one of which is terrestrial), while XM has only one. As I mentioned in another thread, at some point in the last 5 years, my Sirius technology radio went from very frequent dropouts (under every overpass), to almost none. I can now listen to the radio on the upper 5 levels of my parking garage a work, and only lose signal in the deepest part of the bottom level. I claim the radio switches to terrestrial transmitters when it can't see either satellite. There is also fancy syncing technology in my radio to allow it to switch between signal sources without dropouts.
While that may have been the case when each had significantly different programming and quantity of channels, I'd imagine they are pretty comparable now given a near mirroring of programming.
Oh yeah? How would any of us know if we don't know what underlying technology these radios use?
I remember when audio CD's first came out and always thought they sounded tinny compared to vinyl records and FM broadcast.
Yup; I remember that too. Jazz labels were making beautiful early CDs, but pop/rock labels were just ghastly. I gather they had to crank up the highs (e.g. hot microphones) to optimize for vinyl (which actually liquefies at the point of needle contact). Pulling out just one part (vinyl record) and inserting digital CDs without re-optimising end-to-end, left many of these early releases with very harsh highs.