As a guy who's used various devices (multiple mp3 players, laptop and sat radio) with fm modulators, I always recommend hardwired audio, if you have the option.
Listening to an aftermarket radio with either a "sat-ready" line-in (which is usually just a line-in 1/8" stereo jack) compared to an fm broadcast from your modulator is a huge jump in audio quality.
That being said, the Sirius model with built in FM modulator is the best quality for no fuss and low cost. If you're swapping to an after-market radio, and you know it's Sirius all the way, then drop in an integrated Sirius/AM/FM/CD deck and you'll get the best audio & display options.
Hope this helps,
Joe