i've had my 430 for over 3 years and the only problems i have had is one parking sensor not working - door speaker blowing - fixed cheaply (£12) and the amp packing up which cost £285 to get repaired. other than that all the electrics work including the steering motors - no suspension problems either.
Thats not to say the ls430 does'nt have it's weak spots, - it does such as radiator, suspension an steering motors.
The Mark 4 400 is a very reliable car ( I owned 2) - but its not as refined as the the 430. I think everybody has different preferences, expectations and attitudes therfore will have a varied judgment of the two cars.
My advice is take a lengthy test drive in both preferably one after the other and that way you will be able to compare and decide between the both.