Love my RX450h, best of everything. Very comfortable with loads of power and good economy. Haven't driven a 400h, but am sure you will enjoy whichever you choose. I would try and buy a car with Lexus warranty on it as it is comprehensive and the dealer can get anything authorised pronto. I commented last week when my car was mot'd that the reverse camera was slow. New media unit ordered which would have cost nearly 2 grand without warranty.
Good luck in finding the right car.