What makes an '02 Seville STS better than an '02 Eldorado ETC? What are the facts? I know the Seville is on an updated chassy - does that mean it handles better? I hope somebody will take the time to explain any differences - besides the obvious...

it's not an updated chassis rather it's an updated suspension. Both STS and 50th anniversary vette came out with the newer suspension which is supposedly more responsive than any other system out on this earth by a margin.