Ok, Firstly I should say 2 things.
1) I sell the Aufero
2) I've never used Back2Natural.

But, the difference seems to be that the B2N is something you need to use regularly to maintain the colour and its best to use it from day one.

Whereas the Aufero is a treatment that can be used once the system has faded and turned brassy and it will remove those tones and restore the original colour.
You can, but don't need to use it regularly.
One treatment lasts me 2-3 months.

I just had an email from a customer that said it restored the 6 month old dry hair back to its original condition.
He reviewed it here:
http://www.hairlosshelp.com/forums/messa...erthread=y

Also, I had a customer, thats used both products, say that the 'base' colour is different.
He said that the B2N is more green based and the Aufero is more purple based.
He felt the purple neutralized the brassy tones in a more natural way, but recommended the B2N to anyone whose hair naturally had a greener ash tone to it.

So personally, I think you should choose the product that best suits what you need to achieve as they work in different ways.