Performance
With that kind of horsepower, both the Porsche Boxster and the Jaguar XKR are legitimate speedsters.
Regarding horsepower, the XKR packs quite a punch and will knock the Porsche Boxster down a few rungs. On the flip side, keep in mind that high horsepower engines often command higher insurance premiums.
Utility
The Jaguar XKR will most definitely prove friendlier than the Porsche Boxster for those of you with frequent child-cargo.
Dimensions
The Porsche Boxster's tank is almost the same size as the Jaguar XKR's, though the Jaguar XKR's is a bit bigger.
Cost
The destination charge is a standard charge for transporting the vehicle from its point of origin to the dealer. It costs roughly the same to get the XKR to the dealership as the Porsche.
The Porsche Boxster is tremendously less expensive than the XKR with respect to MSRP.
Warranty
The Jaguar XKR and the Boxster have the same basic warranty.