i'm not a fan of xm, and i like the idea of being able to walk around with every song that you've ever liked, in the palm of your hands...so i would have to go with the ipod...but get an aftermarket software because that "itunes" crap SUCKS!!!!
The Sirius walk around will not receive Sirius programming unless it's plugged into a dock, meaning you can't walk around and receive live programming. Sirius is useless unless you can see the sky anyways so it doesn't matter. The myfi (XM) is cheaper and will work walking around but has a much smaller hard drive.
The ipods don't have any kind of built in tuner so it's mp3s, videos, m4ps or nothing. The Creative Zen Vision is the answer Massive storage, does everything the new ipod does and more (has an FM tuner), bigger display and a CF card port for moving files.