My dad had an STS before he got his LS400.....
There's no comparison, the LS400 is 100% more car. Ride MUCH better than the Caddy, RWD vs FWD, better powertrain, MUCH higher quality, MUCH more reliable, better performance, more prestigious image. Its just no contest.
He says it best "After driving the Lexus, driving a Caddy again feels like driving a Chevy".