At this point, there isn't a huge amount of difference between Sirius and XM, even though their networks are still separate (until a few years ago, they used radically different satellite constellations: XM and now Sirius are geostationary from 2 satellites, Sirius previously used a tundra orbit with 3 satellites (only 2 transmitting at a given instant)). The two platforms encode audio differently (Sirius uses PAC, XM uses an early iteration of HE-AAC): XM's is generally considered to offer better sound quality at most bitrates (though I suspect that PAC uses a couple of techniques to make sound quality more gracefully degrade when there just isn't the bitrate available; PAC also seems to handle bass-heavy content better, which explains some historical audio-engineering choices made on the two platforms), and the subscriber authorization schemes are radically different (XM continuously reauthorizes, once a Sirius unit authorizes, it stays authorized until it gets a kill signal).

The longterm plan is to eventually decommission the Sirius network (most of the OEMs have stopped equipping new cars with Sirius units, with Ford being the last major holdout). The SiriusXM-branded radios are essentially XM 2.0. So far the next-generation combined streaming (including Pandora-powered custom channels) and satellite platform (called SiriusXM 360L) is only in the new generation Rams and there's an aftermarket unit available in a few months.