XM radio most definitely is a "sure thing". At least right now it is. Hell, there is a billion dollars riding on it! No doubt, I firmly believe one of the two digital radio services will go under. I am betting on Sirius because they are so late to come to bat.
As far as the antenna, I would get the Terk "hockey puck" style antenna and mount it on the dash between the two defrost vents. That is where I had my GPS antenna on my Supra.
Another option is between the glass and the rearview mirror mouting arm. You will need to build a "mouting" plate to hold the antenna but it is not too difficult.