I got a GPS w/ XM Navtraffic and decided to give XM Radio a try (I've had Sirius for ~2.5 years now).
My Sirius tuner is a Kenwood tuner hooked up to the back of my HU. It displays nicely on the screen. I like that. It also sounds nice and clean. I bought a lifetime subscription and never really thought about it. I figured, "Ok, they both play music. XM supposedly plays deeper tracks - doesn't really matter to me."
So I get the GPS, give XM a try (mainly because I like some of their talk radio lineup (Bruce Williams, Coast to Coast AM, etc.) and listen to the HitList (XM30). Man, that channel is great. Way better than Sirius Hits 1. Now, a more comparable channel on XM for Sirius Hits 1 would probably be 20on20 (XM20).
I've never been a die-hard Sirius fan other than for the:
* Hey, it's better than FM
* They offer lifetime subscriptions - pay once and no more bills (for that tuner)
So, anyway, I've got a few beefs with Sirius where they've tried to sound more and more like FM. Their DJs are overly chatty. I don't mind DJs as it kind of breaks things up. I really don't care about what they had for breakfast or the Hollywood gossip of the day. The free Sirius radio promotion before Christmas was horrible. Every few songs I'd hear about how great it'd be for me to pick up Sirius for friends or family and that way they too could listen to commercial-free Sirius radio (well, as long as you ignore the Sirius commercials and the self-promoting nonsense).
So, anyway, getting back to XM - I'm driving around listening to it (HitList) and noticing that they're playing more than the top 20 (which, I guess I could find on the 20on20 channel) but it was great hearing some songs from the last 3-4 years mixed in with some more recent hits. Also, the XM DJ would come on every so often and just give a quick blurb about something. Not intrusive at all. Once in a while I heard something about 'don't forget to check out XM Sports' (or something). Anyway, it was way less than what Sirius was playing for their self-promoting stuff and the DJ didn't tell me about what he had for breakfast.
Basically, I picked up Sirius because it worked with my HU (at the time you didn't have a choice once you had the HU) and it offered lifetime. If I were to do it again today after listening to both, I'd definitely pick XM.
The self-promoting stuff really stinks. I keep thinking, "Don't tell me you're great - just be great and you're not being great if you're telling me you're great."
As it is, I'm going to keep the XM subscription in my GPS. Oh, it's hooked through the AUX-In on my HU but doesn't sound quite as good. This could be because it's primarily a GPS and XM was and add-on feature for it. I'm not sure. Anyway, XM will be getting my money. I'll still have Sirius to listen to, though, since that one is already paid for.
I'd definitely recommend giving XM a listen for the next 3 months before you decide on going Sirius in your next car. You already know how they sound. In 3 months, if you still think, "I like Sirius better," then you've made an informed decision.