I haven't driven the GS450h but have the IS-F, and if it was me and those were my only two choices it would be the IS-F every time.
It is a great car to drive, and has enough by way of luxury for me. Although coming from an LS you might have different priorities, I don't think the IS-Fs ride is harsh at all, but it does get a lot of abuse in reviews about it.
The GS450h is very brisk in a straight line, won't be as good in the corners but has to be more comfortable due to more compliant suspension. Plus the GS450h Sport has active roll bars which might make it decent enough through the corners.
You're probably going to have to try both and see which you prefer - which doesn't seem like a bad way to spend an afternoon :winky: