From what I gather, integrated Sirius/XM systems are wired directly to the antenna input on your car stereo, which means that a wireless FM transmitter could not be better. But it could be close to the same, I suppose. You ight be better off either getting a stereo with an AUX input, or wiring your own AUX input, and then use a cable from the headphone jack of whatever you want to listen to (iPod, BB, etc).
I'm almost positive that some stereos have bluetooth capability, so that's another option. With the limits on consumer-level FM transmitters imposed by the FCC (they used to be more lax, but recently, manufacturers have had to seriously weaken the output strength to comply with the law), I think we'll see less and less of them over time until they're essentially obsolete. If you can buy one and tweak it to broadcast at a higher strength, then that might work. Also, some cars are better suited to accepting weak signals. In my Corolla, I have static with my iTrip on all bands, and all locations in my car. But in my dad's Lexus, I can put the iTrip anywhere and get a crystal clear signal.
Bottom line: there are too many variables to say whether or not an FM transmitter will work up to your standards.