I have had both and would recommend the RX450h simply on the basis that it is more up to date, quieter (double glazing / laminated side windows help!) and more refined. Don't forget that the RX400h is a design that dates back to 2003 (in RX300 guise).
If you go for a 400h, try and source the outgoing model Executive Limited Edition, which had all the toys.
If you go for a 450h, I would personally stay away from models with the air suspension. There is no noteable difference with the normally sprung car, but repairs to the suspension can be prohibitively expensise, especially considering that you will be buying a 7 year old car and keeping it for a further 7 years. I would also recommend budgeting for an extended warranty.
As per Raayan's comments, I would not worry about mileage.