I guess the choice really boils down to Symphony II or RNS-E. In my stable I have both, with BOSE.
The Symphony is easily available and inexpensive, and gives a great sound with CD's. One small downside, it will need coding for BOSE through VCDS if not already done.
The RNS-E doesn't need special coding as I understand it, it's more intelligent and recognises when it's connected to a BOSE configuration. It gives an even better sound quality as it has an internal pre-amp. I'm sure you'll know about the MP3 card slots and of course there's the sat-nav. You also can use a dedicated AUX-In feed with a late enough firmware level, instead of a CD-changer emulation device. On the downside, to get the most out of it you need DIS and a rear 6CD changer. And then there's the price, many times what you'd pay for even a very good condition Symphony.
If like me, most of your music is played through an AUX cable and you don't feel the need to use Nav much, the justification for the RNS-E is difficult. But once it's in there, you'll be reluctant to take it out again.
EDIT: Subsequently seen
this Timmus post where he mentions that the RNS-E
does need coding for BOSE, plus wiring adaptation to the Quadlock adapter. I stand corrected.