Depends.
Being an enthusiast forum, ofcourse all previous responses are 'S4', but the reality is there are a lot of PROs/CONs to each choice. Obviously if your primary concern is performance, by all means get an used S4. But if you don't 'need' S4 performance, a new A4 can be configured any way you want, any color combo, does have newer tech, better MMI, nicer dash/console, is new and unabused. To me one of the biggest obstacles is finding that 'perfect' pre-owned S4. If you are specific about colors, options, etc...it can be hard to find one that meets those desires AND is the right mileage, condition, possibly CPO, etc.
From my experience with several A4s/S4s of different generations, they're both great cars, and a lot more similar than most people like to believe. The S4 has a bigger motor, which is the key difference. You can argue all the other differences and how tangible they are or are not, but power is what it boils down to. The gap between A4/S4 performance and appearance has declined significantly over the generations too.
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