The interior styling of the ES and GS are very different. I have only driven the ES as a loaner when my GS is in for service. Personally, I'm not a big fan of the current model ES interior. To me it just feels more like a standard Toyota. The GS interior has that extra something that makes it feel more luxurious.
The exterior is also very different. The GS, looking at the front, has much more of a sporty look to it. In fact, the lines on the GS are really all about a sporty look, compared to the standard sedan look, which the ES has.
But, the most important difference for me is the AWD. I live in Minnesota....snow baby snow! I won't purchase a car without AWD anymore. The GS does a nice job in managing the AWD system. In warmer weather it is balanced at 70% rear and 30% front, which gives it a sportier driving experience. When the weather turns cold it is 50/50 rear/front.
--Duckman