Well as all the techy questions seem to have been adequatley answered, I think some aesthetics are called for. My Wife and I fell in love with the IS200 and when we saw the sport version there was no option than to fork out the extra.
We couldn't wait to get our first Lexus, and the feeling I get when I'm driving it (or looking at it) is just out of this world. In fact it's the only car we've ever WANTED to drive at every opportunity. Buying the sport, I don't really feel obliged to invest in add-ons, it's just perfect as it is.
Enjoy what ever Lexus you get. I'm sure you will.