Well I know what I would be doing, if both cars check out good and they are close in price I would buy the GS430, you get the V8 4.3 L with 300 HP, and they aren't that much different in fuel costs. Both cars are great vehicles but I like my power. The other thing is that the GS430 might come with bigger wheels and tires depending on factory options ordered when new. The GS430 should have the trunk spoiler, some GS300 have them as well. The GS430 will have a couple of different options inside as well. Overall your buying a great car which ever one you purchase. Good luck with your purchase.
My 2 cents....
Dave