I used to drive 2002 gs300, and after a rear-end accident 2003 gs430. In gs300, the average was 18-19 mpg. For gs430 it's 19-20 mpg. Both are for 50/50 city/hwy. In gs430 the mpg is on the center console display and I can pay more attention to crusing through busy traffic, thus save some gas. On freeway, gs430's mpg are better than gs300.
Also, when flooring on the gas paddle, I can feel .5 second of initial delay on gs430, whereas gs300 would just speed up and go, so gs300 feel more agile in that sense. However, once get past the delay, gs430 has *unlimited* power. Gs430's engine roars, gs300's engine screams. Both sound wonderful.