Now here's a stupid fuckin' idea.....why not take them both out on a proper test drive and see which one you prefer???
I have had the 335i and I was looking at an S4 avant, now to put this in perspective I had an M3 SMG coupe and an A6 3.2 avant at the time after wanting another M3 again as the 335i was nice, fast, but a little dull. Also my 2005 A4 avant was the worst car I have ever owned, hated it in so many ways it is untrue, nose heavy, poor gearing, nasty clutch, and just horrible to drive in anything but a straight line.
However I bumped into a someone from one of the magazines one evening and he had the S4 avant press car, with S-Tronic and just about every option you could tick, he had been driving an E92 M3 previously, and he said the new S4 was great, a car he would probably spend his own money on.
I decided to take the S4 out, managed to get it for the whole day which was a bonus.
Firstly the interior is great, couple of odd things like the way the air vets are different heights from passenger, middle and drivers side which makes the cabin look awkward, and for some strange reasson the cd player is high and the climate low leaving a black slab of plastic in the middle of the dash, but overall it is a nice place to be. The pedals are better positioned than the previous S4 and the seats are much nicer, the old car looked like it had nice seats but they were not great considering they were Recaros, I think the under thigh support raises more on the new one, more like the BMW sport seats.
The new MMI system is pretty nice, although not actually as good to use as iDrive, easier to learn but using it you have to delve deep into menus to do the simplest of things, like call home.
But all in all a good infotainment system.
The car actually feels similar to my old 335i, just as quick, but smoother, more refined (which to me is not a positive) but as quick, however the first thing you notice is the steering feels more artificial, even setting it to the sportiest of settings the car doesn't have that balance you have with a BMW, don't get me wrong, the best all round Audi ever made after the RS4 (not tried the R8) and I would take it over the RS4 as I prefer the DSG 'box.
But going across the twisties it just didn't reward you in the same way the 335i does, it does it all in a very cultured relaxed manner, no drama, no fuss, it just takes you through the bends. One problem is you can't feel what the car is really doing beneath you, each time you do a stretch you can get faster and faster, you get to remember how fast you can do each corner, but that is not the same as feeling what is happening and reacting on the fly, at least with the BMW you can tell when it is going to let go.
All in all a really nice car, and one I would definitely own, for me the 335i always felt a real step backwards from my M3, but then it was a CSL so was in a different league feedback wise, but then after selling the CSL and getting the 335i I went back to a regular SMG M3 and still thought it was 100x better than the 335i in every area other than outright pace (my 335i was remapped so quicker.)
So I wouldn't actually want either, I would have the S4, but in avant guise and as my second car, it just isn't fun enough, if you want a fun car buy a used M3, if you need a sensible car that can gobble the miles in complete comfort and far faster than most things out there then the S4 is great, if you want something that is going to make you want to go out and drive just for the sake of it then get an M3.