I know an old post but wanted to add my 2cents to anyone comparing the two. Especially now due to increased popularity in London as we have the imminent ULEZ expansion.
I own both RX400H (2005) 215k miles & RX450h (2011) 75k miles
I prefer the boot space in the 400h. It has several extra compartments under the boot floor as the spare full sized wheel is under the car.
The rx450h has none as there is a space saving wheel under the boot floor instead.
I found the extra space in the RX400h compartments invaluable for all the loose items worth keeping permanently in the car, such as; spare jump start pack, first aid kit, wheel lock nuts, torch, cutlery/cups, kids play items, hats & gloves & scarfs, medicine, small sports items such as frisbee, pet items such as dog ball, dog lead etc..
This leaves the boot completely free for boxes to hold your regular shopping or other large items.
It's just one of many reasons Im keeping the 400h as it's more practical for everyday use.