I think the deciding factor will be whether you want a cradle dock. From what I have been reading, first-time users should probably get a cradle because it is easier to use. I think the Sportster has a cradle.
I love my Starmate Replay dearly. I haven't had it properly installed yet though so I can't comment on ease of use. Once I get it installed, I will have to pull 2 chords out every time I unhook it. The cradle dock you just slip in.
But by not having a cradle, you have more flexibility in where to put it. Check out the Sirius Backstage forums installation section for some ideas.
So I am not sure which one to suggest for you lol. I guess it's however you want it to hook up.