Cost
The Lexus IS-F will tax your gas budget significantly more than the GS 450h will.
When it comes to fuel economy, the Lexus GS 450h is a bit better than the IS-F, but not by much.
Regarding MSRP, the GS 450h doesn't cost much less than the IS-F.
Utility
Drivers with frequent kid-duty might favor the GS 450h over the Lexus IS-F.
Convenience
The Lexus IS-F's fuel tank will need to be replenished more often than the unit in the Lexus GS 450h.
Comfort
Taller drivers will considerably enjoy the extra head room in the IS-F's front cabin compared to the Lexus GS 450h.
The Lexus GS 450h offers more head- and leg-room to your passengers than the IS-F does.
Dimensions
The Lexus GS 450h is significantly more bulky than the IS-F.
The IS-F's tank is almost the same size as the Lexus GS 450h's, though the Lexus GS 450h's is somewhat larger.
The Lexus IS-F and the GS 450h compete for the same parking spaces.
Performance
Regarding horsepower, the IS-F packs quite a wallop and will knock the GS 450h down a few rungs. On the flip side, remember that high horsepower engines often command higher insurance premiums.
Handling
With its tighter turning radius, the IS-F is definitely more agile than the Lexus GS 450h, something to consider if you do a lot of city driving; on the other hand, keep in mind that a vehicle with a tight turning circle may feel a bit twitchy on the highway.
Drivetrain
The Lexus GS 450h's engine is considerably smaller than the Lexus IS-F's. Remember that smaller engines can be stressed more; on the other hand, smaller engines are often cheaper to fix.
The Lexus IS-F packs quite a punch compared to the Lexus GS 450h as far as torque, which is roughly equivalent to acceleration.
Torque is the force that lets you accelerate quickly, and in this respect, the Lexus IS-F is significantly more powerful than the Lexus GS 450h.
Warranty
The IS-F and the GS 450h have the same basic warranty.