I've driven the GS430, and I must say I was v.impressed. But im only 21, the GS's time will come with age :D
Both lovely cars, especially the new GS B) but at the mo, my vote goes to the IS300!!
Difficult to compare the two, GS is a softer, luxury drive I spose whilst the IS is a drive and look good car, and it pulls it off well!!
My advice to you - test drive them both. More than once if unsure. If you're still undiced go for the GS, think of the IS as an old legacy and the GS is the next chapter. LS is next though!! :shutit: