I drove the S5 as well. It was a great car but felt a lot heavier and less agile than the 335i did to me. The interior was nice but a bit too plain jane.
But there were 3 main reasons I wasn't interested in the S5:
1) The bodystyle is due for a refresh in the next year or maybe two.
2) Lease rates and residuals on Audi were really bad and made for a much more expensive car over the lease term than a more expensive MSRP on a BMW.
3) Reviews have said that Audi Drive Select is a must have option for the S4 and when you add that the car is at $60k. That with the issue of #2 above made a super expensive car to own.
I have also owned a G37 before and sold it after a year. I actually thought it was a great looking car but what an abysmal engine it had. 330hp? Really? Where? Seriously felt like 240 until you get it to 5,000 rpm then it kicked in for a little bit and seemed to quickly drop off.
Mercedes just wasn't appealing to me so I didn't even consider anything they make. I also didn't care for the look of the Cadillac, too boring to me and really am not prepared to plunk down ~$50k for a GM product.