1. A4 Prestige, you can only add S-line to the Prestige model. Which means the car is fully loaded. If you want an A4 Sline, you're getting a fully loaded car.
If you price an A4 Prestige Sline Manual, you're at a $47K MSRP, very similar to a 328i Xdrive M Sport Package with Technology package and Real Dakota Leather, you're over the $48K MSRP already. No backup cam, no heated seats, moonroof, permium sound system, park distance control, blind spot, and most importantly XENONS with the BMW.
No lets compare the 335i XD and S4:
S4 Manual Premium Plus w/ Navigation Package + Carbon Fiber Inlays $52.5K
To price a comparable 335i XD to be somewhat similar to what the S4 offer u need to opt for the technology package ($3100), Premium Package ($2200), Heated Front Seats ($500), Rear Camera + Distance ($1150). ----$56.3K
Now I know a lot of BMW owners would prefer the rear wheel drive of a 328i or 335i. I don't blame u. If you don't need an AWD car (geographic location), why pay extra. Which is great because you can order your car al la carte. Audi just packages everything together, a different strategy, whether you want the extra goods or not its not up to you. Budget conscious minded buyers will most likely go with a BMW. Another reason why we so many on the road (at least in Los Angeles).
Like some have mentioned, in the end, the choice is YOURS. Its good to have options. Its good that we have companies competing against each other. To compare these models is not far reaching at all and its almost a must when you're shopping. Whatever you end up getting, I'm sure it was catered to the BUYER's WANTs and NEEDs.