ORIGINAL: QuattroGT74

I have an 05 STi and an 06 S4, so here is my 2 cents.

The STi produces louder tire noise, interior noise, rattles and squeaks etc than an S4.
The STi has a unique exhaust note for a T4 but the V8 is intoxicating on the S4, but it still does not interfere with the interior.
The S4 is firm but is a Cadillac in comparison to the ride of the STi.
Ave mpg on S4 = 16mpg and on the STi 18mpg and that is driving with basically every shift going to redline. However I still have very few miles on the S4 so take that into account.
The handling on the S4 is great but pales into comparison against the STi of course. That level of grip is tough to beat.
The S4 feels heavier than the STi (as it should) but the build quality, tightness and feel is far superior on the S4 obviously.
The DCCD is great but I still prefer Quattro overall.
The S4 is quick but you will certainly miss the performance of the STi.

Both are great cars and I am fortunate that I don't have to give either one up, because I have no idea which one it would be. However, considering your needs I think the S4 would be a very strong choice for you. Good luck.