There is deep, and there is obscure.
In my opinion Sirius is deep, XM is obscure.
XM goes into obscurity and plays bands I never heard of, songs I never heard of, and quite frankly, I didn't like it then, and don't really like it now. Now a lot do like this. If you do, and music is your #1 priority, I recommend XM.
Sirius goes deep, and got even deeper around mid december by expanding playlists. I hear songs that I love, but maybe haven't heard for a really long time. Or I hear songs from bands that I love, but the typical hits. But I rarely hear an obscure band that I never heard of.
There are exceptions, but thios seems to be the pattern. Take a channel like the vault. I hear songs on there that I have never ever heard on FM and probably never will, however the bands I hear are band like Clapton, Floyd, Petty, Stones etc.. just deeper stuff.
Take a way overplayed band like Metallica. Yes Sirius probably plays way too much metallica. However the majority of the time it is sutff like "Fight Fire With Fire", "Whiplash", etc... Stuff I've rarely ever heard on the radio, have forgotten about since I listened to those albums 20 years ago, and I hear today, and say, "Man that song rocks. I remember that song"
Where with me, when things get obscure, I usually change the channel. I listen mostly to musci from the 80s and 90s, and it's rare I ever hear anything obscure, that I didn't like back then, that I'll enjoy now.
Others want the obscurity. Hell you can try them both for free on the internet. Give it a shot.