Performance
With that kind of horsepower, both the Porsche Boxster and the 650 are legitimate sportscars.
Handling
With their similar turning radii, the Porsche Boxster and the BMW 650 most certainly handle roughly the same into and out of tight spots.
Drivetrain
The Boxster's engine is tremendously smaller than the BMW 650's. Note that a smaller engine usually has to work a little harder; on the flip side, smaller engines typically get better gas mileage.
The BMW packs quite a punch compared to the Porsche Boxster in terms of torque, which is roughly equivalent to acceleration.
Torque is the force that lets you accelerate quickly, and in this respect, the BMW is tremendously more powerful than the Porsche.
Utility
The BMW 650 will most definitely prove more convenient than the Porsche Boxster for those of you with frequent kid-cargo.
Comfort
While the front cabin in the Porsche Boxster offers a bit more head room than the 650, there frankly isn't much of a difference.
Dimensions
The 650 is tremendously more bulky than the Porsche Boxster.
The Porsche Boxster's tank is approximately the same size as the BMW 650's, though the BMW 650's is somewhat larger.
The Boxster and the BMW 650 compete for the same parking spaces.
The Porsche's shorter wheelbase may produce a less-settled highway ride but a more agile feel in daily traffic than the BMW 650.
Cost
The destination charge is a standard charge for transporting the vehicle from its point of origin to the dealer. The BMW 650 is considerably cheaper to get to the dealership than the Porsche Boxster.
In terms of MSRP, the BMW costs tremendously more than the Porsche.
Warranty
The BMW and the Porsche Boxster have the same basic warranty.