Performance
With respect to horsepower, the Toyota Avalon packs quite a punch and will knock the Lexus IS 250 down a few rungs. On the flip side, note that high horsepower engines often command higher insurance premiums.
Handling
With its tighter turning radius, the IS 250 is quite certainly nimbler than the Avalon, something to consider if you do a lot of city driving; on the other hand, note that a vehicle with a tight turning circle may feel a bit twitchy on the highway.
Drivetrain
The Toyota Avalon has a larger engine than the Lexus IS 250. Keep in mind that smaller engines typically get better gas mileage, but a smaller engine usually has to work a little harder.
The Toyota packs quite a wallop compared to the Lexus as far as torque, which is roughly equivalent to acceleration.
Utility
The Toyota Avalon and the IS 250 can accommodate the same number of passengers.
Convenience
You most certainly won't have to visit your friendly gas station more often with the Lexus IS 250 than with the Toyota Avalon, as both offer comparable mileage ranges.
Comfort
Taller drivers will significantly enjoy the extra head room in the Toyota Avalon's front cabin compared to the Lexus IS 250.
The Toyota Avalon provides more head- and leg-room to your passengers than the Lexus IS 250 does.
Dimensions
The IS 250's tank is approximately the same size as the Avalon's, though the Toyota Avalon's is a tad bigger.
The Lexus IS 250 and the Toyota Avalon compete for the same parking spaces.
Cost
The destination charge is a standard charge for transporting the vehicle from its point of origin to the dealer. The Toyota Avalon is substantially cheaper to get to the dealership than the Lexus IS 250.
The Toyota Avalon doesn't pale in comparison to the Lexus IS 250 with respect to fuel economy.
As far as fuel economy, the Avalon and the Lexus IS 250 don't differ much.
The Lexus IS 250 isn't much more expensive than the Toyota regarding MSRP.
Warranty
The basic after-sale protection that comes with the Toyota Avalon isn't quite as generous as the Lexus IS 250's.