I've never used a Garmin solution so can't compare the two, but am happy with the Tomtom solution that I use (v2 on an iPAQ 5450 using a Fortuna wired receiver).
For a receiver, the Tomtom B/T receiver doesn't get the best write-ups in the world, and being silver can cause annoying reflections on the screen. The Fortuna Clip-On gets good press if you're looking at alternate B/T GPS solutions. You may find the pocketgps site useful, especially the forums there and also the comparison tables (especially
http://www.pocketgpsworld.com/compar....php?TempID=15)
If you manage to find a solution that uses RDS then I'd be really interested, but I understand that the RDS process requires a specific paging signal and I'm not aware of any system that'll do this. However, there are plenty of mounting solutions available, including some with amplifiers built-in, and both Seidio and Holux may have products suitable for your need. There is one other option, which is to use the bluetooth connection to a personal headset. However, two points to note.
1) I understand that Tomtom 3 uses a B/T driver that locks the serial port for B/T to the GPS mouse so you can't use another B/T device - I'm told an alternate driver is available but I don't have a copy
2) You would need to be sure that the XDA could cope with such a solution, I believe you need an "Audio Gateway" profile on the device to enable it to work this way - see also the write-up on the iPAQ 6315 on the pocketgps website.
Good luck, Al