XM vs. Sirius Portable Radios.....?

I am considering not buying another iPod for working out in a gym, but rather an XM or Sirius portable radio. I am in a metropolitan area and will mainly use it within the confines of a gym. Which portable radio will work best for me? Basically, I want a clear signal in doors with as many comedy stations and sport stations as possible. Which type and brand of radio would best suit my needs?