Go with XM for Oldies
I'm with thefonze on this one. Even though Sirius picked up some of the WCBS-FM jocks, if you want original oldies music than you're better off with XM. I have both XM and Sirius since I ditched AM/FM years ago. XM has a much deeper and more obscure playlist and their shows between 50s on 5 & 60s on 6 include "Harlem", "Rockabilly Road Trip" "Sweet 16 countdown", "Pink & Black days", "Radio London" just too name a few.
I dont know if Sirius will up their playlist with the adding of new DJ's but as of present they keep the usual FM 500 song playlist on both of their oldies channels. Its a common discussion on the sirius boards and most agree Sirius falls short in this area of programming. Sirius' 70s station beats XM's in my viewpoint, but XM's 80s on 8 is hands down better than Sirius.
As someone who has both XM and Sirius whom is also a long time oldies listener; I would choose XM over Sirius if you have too decide one over the other.
Regards,
Anthony--