IMNSHO the GS450h is 'WAY over-powered and a horrible prostitution of the use of the HSD concept. I am normally an advocate of RWD (as opposed to the "other" choice) but it seems to me that the absolutely STELLAR level of low-end torque provided by the GS450h's electrics would be really hard to tame absent a very quick reacting TC, Traction Control, system.
So be prepared to either switch TC off or spend a lot of time "feathering" the throttle in attempting, I suspect mostly unsuccessfully, to defeat TC.
I also consider the RX400h just another prostitution of the HSD concept, TOO much power, unwarranted power, for the platform. So I would recommend(***) the RX400h F/AWD model even though it is mostly FWD, and even with F/AWD remains most definitely FWD biased, be a better choice for adverse wintertime roadbed conditions.
Should you go ahead with a GS450h purchase I would recommend NEVER traveling without tire chains on board in the wintertime and not hesitating even briefly to install them when encountering adverse roadbed conditions. With the GS450h's dramatically HIGH level of low end engine/drive torque, even with a good, even excellent, set of winter treads the onset of wheelspin (even considering a "snow" mode, engine/electrics "derating" activation) will often be so quick and sudden you will be forced to have TC active at all times.
*** The RX400h can only have tire chains installed on the front. Raising the potential, by Lexus own admission, for loss of directional control substantially. Adding 1.5 inch wheel spacers at the rear will allow the use of most types of tire chains at the rear first, SAFELY, and a second set at the front if the need should arise. I added 1.5" wheel spacers, 17X8 wheels, and wider tread tires all around on my 2001 F/AWD RX300. Slightly wider "stance" improves stability overall.