I chose XM for O & A. I'm even happier now that Ron & Fez have joined the lineup. Also, XM clearly has better equipment. The Roady was great, the MyFi is even better. Also, with all those high priced contracts Sirius is signing how long before they have to start charging $15-20 a month.
My only reservation about XM is that the music tends to be TOO obscure. I'm sure sure music lovers like that, but I'd rather have the music I know without ads and DJs.