You're exactly right with your "to each their own" statement. I had both services while they were separate. And they were different enough that most people would prefer one over the other.
Sirius seemed to group their music more by style... eg, you had an "alternative" channel that also played some hip-hop and metal, because that's what skaters listened to.
XM was always "too cool for the room" music. Lots of b-sides and lesser known music. I liked using satellite to turn into something i've heard before, so this didn't match, but exactly like you said, everyone has their own taste.
An interesting tidbit that I had heard was that XM started by really focusing on the tech. Their radios were smaller, and cooler, at the start because that was their focus. Sirius on the other hand focused on DJs. They hired DJs to "manage" stations, programming the channel with what their experience told them was the best fit. Both interesting ideas but its just such a niche market now with the widespread use of mp3 players.
I hope they find a way to make it work. The best radio show hosts are all owned by SiriusXM right now (Ron & Fez, Stern, Opie & Anthony) and I hope they find a way to use them productively.