Utility
The BMW 135 will most definitely prove friendlier than the Porsche Boxster for those of you with frequent kid-cargo.
Dimensions
The BMW 135 is tremendously more bulky than the Porsche.
Your wallet will hurt more every time you fill up the Porsche Boxster than the 135 because of its bigger tank.
The 135 and the Porsche Boxster compete for the same parking spaces.
The BMW's longer wheelbase will result in a smoother, more stable ride; on the other hand, the Porsche Boxster may give a nimbler feel in daily traffic.
Cost
The destination charge is a standard charge for transporting the vehicle from its point of origin to the dealer. The BMW 135 is considerably cheaper to get to the dealership than the Porsche Boxster.
The Porsche Boxster is significantly costlier than the BMW 135 with respect to MSRP.
Handling
With their comparable turning circles, the 135 and the Porsche Boxster likely handle about the same into and out of tight spots.
Drivetrain
The Boxster's engine is slightly smaller than the BMW 135's, but not terribly so.
Torque equals pickup, and the BMW 135 will have picked up and gone by the time the Porsche finally gets moving.
Torque is the force that lets you accelerate quickly, and in this respect, the BMW 135 is substantially more powerful than the Porsche Boxster.
Comfort
While the front cabin in the Porsche Boxster offers a bit more head room than the 135, there really isn't much of a difference.
Warranty
The BMW 135 and the Porsche Boxster have the same basic warranty.