Cost
The Lexus ES 350 won't save you much money at the pump compared to the Lexus IS 350.
With respect to gas mileage, the Lexus ES 350 and the IS 350 don't differ much.
The Lexus ES 350 isn't much less costly than the IS 350 in terms of MSRP.
Utility
Neither the Lexus IS 350 nor the Lexus ES 350 has a marked advantage in terms of seating capacity.
Convenience
The Lexus IS 350's fuel tank will need to be replenished more often than the unit in the Lexus ES 350.
Comfort
While the front cabin in the Lexus ES 350 offers a bit more head room than the IS 350, there honestly isn't much of a difference.
The Lexus ES 350 provides more head- and leg-room to your passengers than the Lexus IS 350 does.
Dimensions
The Lexus IS 350's tank is roughly the same size as the Lexus ES 350's, though the Lexus ES 350's is a tad bigger.
The Lexus IS 350 and the Lexus ES 350 compete for the same parking spaces.
Handling
With its tighter turning circle, the Lexus IS 350 is most certainly more agile than the ES 350, something to consider if you do a lot of city driving; on the other hand, remember that a vehicle with a tight turning circle may feel a bit twitchy on the highway.
Drivetrain
The Lexus IS 350 and the ES 350 have similar-sized engines, with a slight advantage to the ES 350.
The ES 350 has no lessons to learn in terms of torque from the Lexus IS 350.
Warranty
The ES 350 and the IS 350 have the same basic warranty.