There used to be college sports conferences carried by Sirius that weren't carried by XM. I've never had Sirius, so I don't know if this is still true. Also, the "diversity" channels mandated under the agreement that allowed SXM government-sanctioned monopoly to happen were different on both services at the outset -- again, no idea if this is still the case, but it definitely was true for some period of time after the merger. On XM radios, those channels are operated by Howard University, the Church of Jesus Christ of Latter-Day Saints (Mormons), and a Korean broadcasting outfit.
One difference that definitely continued after the merger was the presence of Clear Channel-programmed channels on XM radios. That was the result of a quid pro quo deal between CC and XM in return for critical financial support from CC that helped enable XM to launch.