The first thing you need to give is a budget, and remember that you can better afford a certain model if you look into buying used instead of new. A used late model M3 will cost as much as a new 3 series.
The S5 and C63 AMG are competing models in the same price bracket, there's no reason you shouldn't be able to afford a C63 but can afford an S5.
It sounds to me like you are looking at:
Audi S5 - Best interior, slowest out of the bunch. I'd pretty much only get this if you're making your purchase solely on how good the car looks, and I agree it's a pretty sexy car inside and out. However I can't say performance wise its much of a stunner as it's the slowest car out of the group.
Meredez C63 AMG - Second worst interior, great sounding engine, second highest hp. I'd get this if you really want raw power and force out of your performance car. Nice big engine that pulls hard starting from low in the power band and will keep you pinned to your seat until redline.
BMW M3 - This car deserves some talking about since it's very peculiar. As moose highlighted BMWs interiors have been lackluster as of late. However this car still has a BMW badge on it so even if the interior sucks it's still gonna have the luxury aspect. The engine on this car is a very interesting piece, despite also being a German engineered DOHC V8 it is very different from the C63 AMG engine. The BMW engine is only 4.0L V8 which makes it much smaller than the 6.3L V8 in the AMG and thus much more lacking in torque (a mere 295 torque rating is about closer to the torque of a Honda Accord than any of the other cars mentioned here) However what makes the 2008+ BMW M3 engine pretty awesome is that it revs to 8000 rpm where you can find 420 peak horsepower. This is the only car out of the bunch I've actually driven and when you get on it, it takes the abuse like a masochist and screams for more. Despite feeling some what gutless, especially lower in the RPM range the car is very fun to rev the piss out of. This car is also one of the best performers when it comes to turning out of the bunch.
Lexus ISF - Pretty solid interior, the most reliable, interesting 8 speed transmission also the cheapest out of the bunch, second slowest out of the group). This is probably the one I would recommend against (only because its nearing the end if its model cycle) However I still think it's a fantastic car. If they were as cheap now as they were when I bought my EVO X I would probably be driving one.
Cadillac CTS-V - At 556hp its by FAR the fastest out of the group, pretty solid interior, but the down side is that it's an American car and there's a stigma that goes along with an American brand. Even though its American 556hp is no fucking joke and this car blows away the rest of the cars in straight line performance so it's my personal reccomendation.
If you are staying away from the performance luxury cars and just sticking more closely to sporty luxury cars your options are:
Infiniti G37 - Would stay away since it's nearing the end of its model cycle, there will be a new model soon, and then you'll no longer be driving a new luxury car.
BMW 335i - Twin turbos are fucking awesome (My personal recommendation)
Lexus IS350 - The 2007-2013 are pretty awesome cars (I regret not buying one in 2007 since they still look good today) but the new model is coming out in 2014, if you can hold off on purchasing until the '14 are available it would be a smart financial decision since Lexuses hold their value extremely well and Lexus wont be replacing that year until 2021 since all their models are updated/replced after 7 years.
Mercedez C Class - boring snoozefest IMO
Audi A5 - pretty sexy car, other than than that I can't really say anything spectacular about it mechanically/financially.