Well, it's a bit subjective really. I personally love the RNS-E Satnav unit, but it is without doubt an expensive unit (probably looking at £300 + for a decent unit). You do get MP3 on SD-Cards, and the Sat Nav is pretty good, even 2019 maps are available.
Without doubt if you've got the BOSE sound system you need to keep the head unit OEM as without it you will lose all the advantages of having the BOSE sound.
The Symphony III unit is a 6-disc CD changer, from A3s around 2005 - 2010, the big advantage of this is it will play MP3 CDs, and the display is much bigger than a Symphony II. But you still need the Audi-Quadlock adaptor.