I was looking to buy a Lexus last Nov. It was difficult to make a decision but one thing I noticed when I test drove the ES vs GS, the ES (that has front wheel drive) has the same engine (350) as the GS, but it's geared differently. The GS has rear wheel drive and when accelerating, the GS has more get up and go vs the ES.
When I was test driving the GS from a on ramp to a highway, it was easier to push the engine to get up to speed and merge. The ES was a little less responsive. As I drive a lot of highway miles, that pushed me towards the GS350.
I hope this helps.
Good luck!
I have an 07 GS350 AWD and the passing power is awesome and I have yet to really push the car. Merging on a highway ramp is effortless, well worth it. The GS comes with bigger tires & high friction front brakes for more sporty performance. The ES is nothing but a luxury version of Toyota Camry. The ES has a cushier ride vs. GS sporty stiffer ride.