I got sirius, and love it. But both XM and sirius offer free 3 day internet trials. so try before you buy.
I mounted my antenna ontop of the rollbar just behind my head. What ever sirius reciever you get should have a Sat. signal indicator, and a very long cord. So just move it around where ever you like and see where the best signal is. The dash sucks cause the window frame gets in the way while traveling north-ish. The hard top, being fiberglass, doesn't hinder the signal much, if at all.
P.S. I got the Canadian subscription, and still get Stern, and all those otehr chanels.