RX 450h has been improved in almost every way. Quality of the interior, quality of the exterior panels, gadgets etc. Sound deadening is superior to the 400h and it handles better. The 400h is more comfortable with serious waft ability but rolls around in corners. The 450h has a better balance imo. Both should be reliable but the 450h would be more so, mostly because it's younger so less chance of perishable items going eg bushings