I think it really boils down to a personal thing.
I've had the mark 3 and 4 of the LS400 and for me the favourite is the Mark 4 LS400 out of the 2
When I bought my 430, initially I had reservations, I think my expectations were that the 430 would be quieter and smoother than the 400 - but it is not - I think noise wise the 430 is the same but due to the wider larger tyres it looses that wallowing effect which when you get used to - you don't notice it - I certainly think if you went straight for the 430 without owning the 400 you would find it very quite and smooth. The gear change is a lot smoother than the 400.
There are other things that for me if I was to go and buy a 400 would be a step back such as
1) Modern ageless looks and cabin
2) Better higher seating position (ls400 are less corseting and smooth that you slide around)
3) Brighter headlights
4) Keyless entry and soft close doors
If possible I would try to drive both marques and then make your mind up.