I will warn you against the 500s bt! I have had exactly that model in my a6 for about 4 months.
It started out great, a lot of functions that see seemingly worked well. It did have problems getting the track listings right on my iPhone, and it wasn't very easy to browse music on my iPhone, but nothing major.
However about a month ago, it started draining my battery over night! It was drawing 5a, I think it was stopping the mmi from shutting down.
So I put it on a switched live, so it was off when the engine was off, that fixed it, but then I couldn't have the mmi on with the engine off. So I contacted Dension support. Eventually they offered me a few software updates.
I did those, and a few days later, the device flashed a few times, and then switched off for good, it hasn't worked since!
I have now managed to get Dension to offer me a refund, but they will only give me the money when they receive my device in its original packaging with its original accessories, and I have to cover the postage to Hungary at over £20!
I have just bought a used mmi module with cables for £200, and all the mmi cables I need to do an install in the arm rest. I will do a write up when that's done. All in with all the cables from Kufatec in Germany, the mmi install will be about £260, and hopefully very reliable!
The whole experience with Dension has really put me off. I think it's a great device with a lot of features, but not very reliable!
Audi told me they used to offer Dension devices as an option, but they stopped after serious reliability issues.