Here is my 2 cents:
1. The B6 will age just as well as the B7. It was designed from the get go to look like that, not a mid cycle refresh. The b6 and b7 both look absolutely amazing.
2. The 40/60 split started before 2007. I think all B7 S4 MANUALS are 40/60 and the automatics picked it up for 2006. Whatever it was, it was before 07, and the autos were at least 1/2 a year behind the manuals in getting it.
3. The B7's do ride better and are quieter inside than the B6's. The B6's heavier steering than the B7's. The B7 feels like a more refined B6 (all it is).
4. My dad averaged 21mpg in his 05 manual. I can't resist that sound (AWE non res) and would get significantly worse.
5. The 4.2 pulls and pulls all the way to redline. It loves to rev. It has plenty of tq for 6th gear highway acceleration, and high gear acceleration without shifting.
Depending on what you want, Jones is right. The 2.0T chipped will give you 90% of the performance, 150% of the gas mileage, and 100% of the looks (with S-Line package or 08+) as the S4. It will get you a newer car, with much less miles and probably even a good chunk of oem warranty, or optional cpo warranty. It would also handle better with a set of coilovers because it weighs a few hundred lbs less.
I have had extensive seat time in my dad's B6 S4, and I owned a chipped B7 A4 and loved them both. I felt my car was easier to drive, felt lighter on its feet, and got MUCH better gas mileage.