As you can see from my signature I am a BMW lover, however you really can't compare the 7 series with the RX. They aren't in the same league.
That being said, I have owned a number of 7 series BMW's including an '01 750il, an '02 745i and an '03 745li. While these cars remain the ultimate driving machine, everything after that falls short in the cars built after the model change in '01. My '02 745 was exchanged for an '03 by the dealer after a year of not being able to solve it's software problems. The '03 was traded away by me after a year of suffering through it's software issues (though not as severe as the first one). Even when the software is working correctly, the i(diot)drive interface is one of the least intuitive systems I have ever encountered. It is cumbersome and distracting. It forces you to go through multiple iterations to accomplish simple tasks (like changing a radio station or the temperature).
However, if you love to drive and don't care about listening to the radio or what the temperature is, then this is the car for you. It handles and accelerates better than any sedan (or I believe you call them saloons) I have ever owned.
On the other hand, the RX400h is the first Lexus I've ever owned and I'm very impressed with the competence of the car. It has a tremdous amount of power, excellent handling (except for the torque steer from the front wheel drive during hard acceleration) and great economy. The software is very well designed (though here in the US they disable many of the phone and navigation functions when the car is in motion which is dumb).
Hopefully, that should talk you out of the BMW purchase. Now if you were talking about an M3 or an M5, this conversation would take a different turn.
