The channels will be 'merging' but make no mistake... Sirius bought out XM. But that's irrelevant.
For now, they're 2 separate companies operating the same way they always have. It'll probably be another 6-8 months or so before major changes take place. The uncertainty is hurting sales so they're definitely eager to fix that.
best thing to do... sign up for a free trial online for both services. XM's web player is far better than Sirius but don't let that be a factor. Simply determine which channels you prefer. Having both, (directv gives you xm) I prefer XMs music choices. Being a fan of Howard, Sirius made the most sense in my car. Music's still good (and for some, might even be considered better... more of a slant towards rock) but in the end, it's purely subjective.