Before I got my S4, I had a B8 3.0TDI, and for daily driving feel, the 3.0TDI is a better drive because it has so much torque low down in the rev range.
If you're looking for actual performance, stock for stock, the S4 is faster.
I'd imagine the A4 3.0TFSI compared to the 3.0TDI would be closer...