Shortly after getting my GS450h, I had the use of a LS430 for a couple of weeks.
My observations:-
The LS430 is quieter and gives a softer ride compared to the GS450h - but I would mention that the LS430 is particularly refined, my previous car was a current model Jaguar XJ, so a rival to the LS' and the Lexus was certainly better in the ride and noise dept...
Also the GS is aiming at a different market sector, which Lexus would argue is looking for a more sporting car, so refinement is less of a priority.
However having said all of the above the GS range and particularly the hybrid is still very refined, equal to that of my previous Jaguar, and when running on electric motor it is all but silent.
The other major observation was from the point of view of design, cabin layout, and equipment. Its clear the GS from all of these areas represents the current Lexus thinking, and as such all looks feels and works in an up to date way, whereas the LS' feels more previous generation - without meaning to offend any LS430 owners

- it seems as though some of the new features were added on wherever possible, but not nesecarily in the most ergonomic position, for example the phone and voice activation buttons were hidden behind other buttons on the steering wheel, and again without meaning to offend in some places felt a little 'Toyota' like compared to the GS which always feels like it was inteneded to be a new generation all 'Lexus' product, which I understaand was always something the designers were very keen to create with this there first of the new generation 'L-finesse - to use Lexus speak' of models.
As I say, I hope I haven't upset any LS owners, after all its certianly a car which is probably as quiet and smooth as some of the best cars in the world.
Regards
Keith