The horsepower is a this thing that majority of drivers pay the biggest attention to. In this case, it is the BMW 6-Series E63 Coupe E63 630i 272HP manual that has by 11 % horsepower more than the Porsche, which makes this vehicle a bit quicker. Because of more horsepower the BMW 6-Series E63 Coupe E63 will as well give slightly better flexiblility in specific speeds.
The width of the vehicle influences directly on passanger's inner room and driving stability. The BMW is 55 mm wider than the Porsche Boxster 987 2.7 245HP. It is often clearly perceptible by those sitting in front, who have more room for their elbows.
In the aspect of fuel tank size it is the BMW which can carry 6 % more fuel in relation to the Porsche Boxster 987 2.7 245HP. The medium fuel container capacity in the sports cars segment, which is represented by the BMW 6-Series E63 Coupe E63, is 68 liters, whereas in S segment (represented by the Porsche Boxster 987) it amounts to 68 liters.
BMW 6-Series E63 Coupe E63 630i 272HP manual has 364 millimeters longer wheelbase than the Porsche. A longer wheelbase gives more stable and comfortable ride, especially when driving fast. It also gives more storage space. But, vehicles having a longer wheelbase are not as maneuverable as those with a shorter wheelbase.
The BMW 6-Series E63 Coupe E63 630i 272HP manual is 473 mm longer than the Porsche Boxster 987. The length of the vehicle plays a role when it comes to driving comfort and room in the trunk. The BMW 6-Series E63 Coupe E63 preasumbly will be offering to it's users more space for legs. Aditionally the BMW 6-Series E63 will be better driven but the Porsche Boxster 987 will be a bit easier for parking because of shorter length of the body.
The higher a vehicle is, the better is the comfort of boarding it and greater possibility of carrying big amount of stuff to it. The difference between these two cars of 83 millimeters in favor of the BMW is small and does not matter much.