If you really had to change the RX350 and didn't want an RX400h due to it not being a proper 4-wheel drive all the time I personally would look at the X5 3.0D. Great diesel M57N engine and good to drive. If you consider a remap you can increase the power and economy too - a lot of talk on the BMW forums about it) The RX range in my personal opinon is great crusing about, but the X5 would be a more involving drive; however it depends on what you are after. I get bored driving my mums RX300 because of its autobox and the way it rolls in the corners (but then I am used to lots of power and harder stiff suspension so not fair to compare) but my mum loves the car dearly!
Also IMHO:
Audi Q7 - MASSIVE TANK and in my local area most seem to be driven by the local drug dealers. I know a few going cheap actually from a leasing company...... a friend of mine buys them to sell on.
Range Rover Sport - Very nice car (when it works!) but again bad drug dealer image.
However, if you are happy with the RX350 then you may as well keep it. :D