I have used Sirius in my car for over a year now (I use the Kenwood FM Mod & controller). It is a good service, but so is XM. Look at the programming in order to make your decision (I think Sirius has more commercial free channels).
If you are upgrading and will utilize the FM Modulation option, you will not get ultra clear reception. It will be as clear as an average radio station broadcast. The best way to get clear sound is to upgrade the head unit to a satellite ready unit. I your car has stock BOSE speakers this may not be very economical (most head units will not match well with BOSE. Consult with a technician on this issue. Good luck!
If you are upgrading and will utilize the FM Modulation option, you will not get ultra clear reception. It will be as clear as an average radio station broadcast. The best way to get clear sound is to upgrade the head unit to a satellite ready unit. I your car has stock BOSE speakers this may not be very economical (most head units will not match well with BOSE. Consult with a technician on this issue. Good luck!