Both are great cars. The Porsche is more of a sports car, and the BMW is more of a grand tourer.
The Porsche uses a B6 (boxer 6-cylinder) horizontally opposed engine. It is very powerful and has a great and unique sound. The Porsche has a better power to weight ratio, and interesting dynamics. The Porsche will be much more sporty, but less smooth, due to its sporty nature.
The BMW has a V8 and sounds very nice. It is more smooth than the Porsche. It handles in a fun, classic way. It is a very sexy car.
Drive both and see witch one you like.