I'm looking to replace my 10 year old IS200 within the next year or so and I'd like to go for the GS, and as pointed out the hybrid ones have a small boot. My option would be to go for a used GS300 (I would want a SE-L spec), but I'm also doing more miles now, so I'm starting to look at used BMW 535d M Sports or used Mercedes E280/320CDI Sports.
I did test drive a GS450h recently and its a rapid car, but as mentioned the boot is small.
I don't buy brand new cars, I tend to buy used cars (say 3 years old) and run them into the ground.